How to perform Linux distribution upgrade?

We run a Univention server that runs based on a Debian Buster distribution. The Debian Buster will no longer be available for security updates at the end of June. We use different servers on Debian, I inherited them from a retired administrator, and I have successfully updated all other systems using these instructions, but this does not work for the Univention server because the command sudo nano /etc/apt/sources.list /etc/apt/sources.list.d/*, there are no references to those repositories buster included that I could change.

What would I need to do to upgrade my server?