Choisir quel kernel doit être chargé au boot : 

1 - Chercher dans le fichier /boot/grub/grub.cfg 

dans la partie 'submenu

chercher $menuentry_id_option'

copier dans un fichier texte le contenue entre " '' " example : 

submenu "Advanced options for Debian GNU/Linux"{
menuentry "Debian GNU/Linux, with Linux 5.9.0-1-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
load_video
insmod gzio
if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
insmod part_gpt
insmod ext2
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root 9142617f-e990-4e30-9754-9504e3c63a9b
else
search --no-floppy --fs-uuid --set=root 9142617f-e990-4e30-9754-9504e3c63a9b
fi
echo 'Loading Linux 5.9.0-1-amd64 ...'
linux /boot/vmlinuz-5.9.0-1-amd64 root=UUID=9142617f-e990-4e30-9754-9504e3c63a9b ro acpi=off apm=off
echo 'Loading initial ramdisk ...'
initrd /boot/initrd.img-5.9.0-1-amd64
}

2 - on conserve et écrit dans le fichier : 'gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b'

toujours dans le fichier temporaire on ajoute '>' à la fin de la ligne ce qui donne dans notre example : 

'gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b>'

3 - chercher $menuentry_id_option pour le kernel sur lequel vous voulez booter, pour ce faire vous pouvez utiliser la commande : 'grep gnulinux /boot/grub/grub.cfg', dans notre example cela donne:/p>

grep gnulinux /boot/grub/grub.cfg
set default="gnulinux-5.8.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b>gnulinux-5.8.0-2-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b"
menuentry "Debian GNU/Linux" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.9.0-1-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.9.0-1-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.9.0-1-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.8.0-2-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.8.0-2-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-2-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.8.0-1-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.8.0-1-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-1-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.7.0-2-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.7.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.7.0-2-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.7.0-2-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.4.0-4-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.4.0-4-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.4.0-4-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.4.0-4-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.4.0-3-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.4.0-3-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.4.0-3-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.4.0-3-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.3.0-3-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-3-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.3.0-3-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-3-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.3.0-2-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.3.0-2-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-2-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.2.0-3-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.2.0-3-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.2.0-3-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.2.0-3-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.2.0-2-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.2.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 5.2.0-2-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.2.0-2-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-5-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-5-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-5-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-5-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-4-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-4-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-4-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-4-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-3-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-3-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-3-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-3-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-1-amd64" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b' {
menuentry "Debian GNU/Linux, with Linux 4.19.0-1-amd64 (recovery mode)" --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.0-1-amd64-recovery-9142617f-e990-4e30-9754-9504e3c63a9b' {

3 - écrire dans le fichier temporaire après le '>' la partie après $menuentry_id_option 

donc si l'on veut booter sur le kernel 5.2.0-2 c est donc : 'gnulinux-5.2.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b', donc cela donne dans le fichier la phrase complète : 'gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b>gnulinux-5.2.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b'

4 - dans le fichier /etc/default/grub on remplacera 'GRUB_DEFAULT=0' par 'GRUB_DEFAULT="<le contenu du fichier temporaire>', donc cela donne dans le fichier la phrase complète : "gnulinux-5.9.0-1-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b>gnulinux-5.2.0-2-amd64-advanced-9142617f-e990-4e30-9754-9504e3c63a9b"'

5 - lancer la commande de mise à jour : 'update-grub'

6 - rebooter la machine