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The apt package tool is used to upgrade the system. You can use the rollback to avoid loss the system (more details in the below section).

Update the database of available packages, with their version:

apt update

Upgrade the system with the new version of packages:

apt upgrade --yes

Upgrade the system with new version of Debian and remove useless packages:

apt dist-upgrade --yes
apt autoremove --yes --purge

Rollback

As mentioned here, the root partition is in LVM disk. With this configuration, it can be save by a LVM snapshot.

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lvremove /dev/VOLUME_GROUP/SNAPSHOT_NAME

More details here here.

Troubleshooting

Can't create a snapshot with old name

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