Upgrade packages
sudo apt update && sudo apt dist-upgrade
Edit sources.list, replace
sudo nano /etc/apt/sources.list
deb http://deb.debian.org/debian bookworm main non-free-firmware
deb-src http://deb.debian.org/debian bookworm main non-free-firmware
deb http://deb.debian.org/debian-security/ bookworm-security main non-free-firmware
deb-src http://deb.debian.org/debian-security/ bookworm-security main non-free-firmware
deb http://deb.debian.org/debian bookworm-updates main non-free-firmware
deb-src http://deb.debian.org/debian bookworm-updates main non-free-firmware
or
sudo sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list
Then upgrade.
sudo apt update
apt list --upgradable
sudo apt dist-upgrade
sudo apt full-upgrade
sudo apt --purge autoremove
Wait sometime. When all is done. Next: reboot server
reboot
After reboot check version:
lsb_release -a
output like
root@deb11:~# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 12 (bookworm)
Release: 12
Codename: bookworm
That's all...