Upgrade to Debian 12 from Debian 11

Make sure that current Debian 11 is up to date with all packages:

sudo apt update
sudo apt upgrade
sudo apt full-upgrade
sudo apt autoremove --purge
sudo systemctl reboot

Verify the status before upgrade:

uname -a
cat /etc/debian_version

Move old APT source file to aother location:

sudo cp /etc/apt/sources.list /etc/apt/sources.list.old

You need to update the /etc/apt/sources.list file by substituting every instance of bullseye with bookworm:

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

Or alternatively, edit new /etc/apt/sources.list file and add the Debian 12 bookworm entries like below, you can remove everything and add those:

deb http://deb.debian.org/debian/ bookworm main
deb-src http://deb.debian.org/debian/ bookworm main

deb http://security.debian.org/debian-security bookworm-security main
deb-src http://security.debian.org/debian-security bookworm-security main

deb http://deb.debian.org/debian/ bookworm-updates main
deb-src http://deb.debian.org/debian/ bookworm-updates main

deb http://deb.debian.org/debian bookworm non-free non-free-firmware
deb-src http://deb.debian.org/debian bookworm non-free non-free-firmware

deb http://deb.debian.org/debian-security bookworm-security non-free non-free-firmware
deb-src http://deb.debian.org/debian-security bookworm-security non-free non-free-firmware

deb http://deb.debian.org/debian bookworm-updates non-free non-free-firmware
deb-src http://deb.debian.org/debian bookworm-updates non-free non-free-firmware

Finally, upgrade to Debian 12:

sudo apt update
sudo apt full-upgrade

After the upgrade is complete, reboot:

sudo systemctl reboot

Check again the system status:

uname -a
cat /etc/debian_version

Cleanup:

sudo apt autoremove --purge