How to upgrade Debian Wheezy to Jessie safely

This tutorial explains how to upgrade your Debian system from Wheezy to Jessie in a few simple steps. Before you start with the upgrade, backup all critical data. Thats especially the config data in the /etc directory and the user data in /home. When you run a server then you should include the databases (MySQL, Postgresql, etc), web and email data in the backup as well.