1. Backup before doing anything! While this is a simple process, things always can go wrong.
2. Confirm the existing installation runs on Debian Buster:
lsb_release -c (out)Codename: buster
root, edit the apt sources by commenting the existing entries and adding the new repositories:
apt edit-sources (out)#deb http://deb.debian.org/debian buster main contrib non-free (out)#deb http://deb.debian.org/debian buster-updates main contrib non-free (out)#deb http://security.debian.org/debian-security buster-security main (out)#deb http://ftp.debian.org/debian buster-backports main contrib non-free (out) (out)deb http://deb.debian.org/debian bullseye main contrib non-free (out)deb http://deb.debian.org/debian bullseye-updates main contrib non-free (out)deb http://security.debian.org/debian-security bullseye-security main (out)deb http://ftp.debian.org/debian bullseye-backports main contrib non-free
4. Update the repository list:
5. Then, upgrade the packages:
Depending on the customizations made in the system, this step may ask to replace configuration files or keeping the existing ones. There's an option to compare the files but when in doubt, prefer to keep the existing ones.
6. If completed without any errors, I'd encourage a
reboot just to make sure the latest Linux Kernel image is loaded as well:
7. And finally, to confirm it's now running Debian Bullseye:
lsb_release -c (out)Codename: bullseye
More details on how to upgrade from Debian 10 (Buster) can be found here.