Upgrade from Debian 11 to 12

Update the existing system first

apt update
apt dist-upgrade

Swap Bullseye (11) for Bookworm (12) in the repositories

sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list

Start the upgrade

apt update
apt dist-upgrade

If all went ok, restart the system

You should then be in Debian 12 Bookworm