How to upgrade Debian 12 (Bookworm) to Debian 13 (Trixie)

Debian version 13, aka Trixie is due to be declared stable 2025-08-09. I ve been using Trixie as testing for a couple of years now and have found it very good indeed. So much so that we are already upgrading our Bookworm machines to Trixie. In all cases so far, this has proven a relatively smooth...