An update broke something. How do I go back?

[MTSistemi]

This is the case Btrfs snapshots exist for, and the system takes them without being asked:
one before and one after every apt operation, keeping five — three ordinary and
two from the important upgrades, the ones touching the kernel or systemd.

[MTSistemi]

Two ways, depending on how bad it is.

The system still boots. List the snapshots and roll back:

sudo snapper -c root list
sudo skillfish-rollback 12

At the next boot you are on the system as it was. The previous one is not deleted — it
is set aside, and sudo skillfish-rollback --undo puts it back if the rollback was not what
you wanted.

The system does not boot. The GRUB menu has a snapshots submenu; pick one and it boots
read-only, which is enough to look around and copy files out. From there the same
skillfish-rollback makes it permanent.

The one thing to be clear about: your home directory is never touched. The system
travels back, your files stay as they are. That is deliberate — a rollback that also
reverted your documents would be a trap rather than a safety net.