Upgrade from Debian 10 to Debian 11

In this tutorial I'll explain step by step how to upgrade from Debian 10 to Debian 11. No data will be lost, of course, and you can keep using the system exactly as before.

First, let's check our version again:

hostnamectl

Next we update everything:

apt update && apt full-upgrade -y

Next, the package cache has to be cleared:

apt clean

After that, we of course need to change the repository. This can be done either via command:

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

or manually in /etc/apt/sources.list:

deb http://deb.debian.org/debian bullseye main contrib non-free
deb http://deb.debian.org/debian bullseye-updates main contrib non-free
deb http://security.debian.org/debian-security bullseye-security main
deb http://ftp.debian.org/debian bullseye-backports main contrib non-free

Once everything is changed, we can update all packages. For a major upgrade the Debian release notes recommend a two-step process: first a "minimal" upgrade (without new packages), then the full upgrade. This reduces the risk of conflicts:

apt update
apt upgrade --without-new-pkgs -y
apt full-upgrade -y

This can take a while. Once it has finished, we can reboot the server:

reboot

Once the server is back up, we check the Debian version:

hostnamectl

If it now says Debian 11, everything worked correctly. We remove the no longer needed packages and reboot the server once more afterwards:

apt-get autoremove -y
reboot

Once the server is back up again, we update the packages one more time:

apt update && apt full-upgrade -y

The upgrade from Debian 10 to Debian 11 is now successfully completed.