I spent some time upgrading the software on the server today. For this, i found a nice reference Bart en David. It is a very nice Dutch manual about apt. Not only does it explain what its all about, fro, a novice point of view, its also usable as reference.
About the author: Bèr Kessels is an experienced webdeveloper with a great passion for technology and Open Source. A golden combination to implement that technology in a good and efficient way. Follow @berkes on Mastodon. Or read more about Bèr.