Upgrading Plausible from 1.5.1 to 3.2.0

I’m one of the (main) sysadmins of a forum running phpBB. It also uses Plausible, a self-hosted web analytics tool, so we have some grasp where our users come from. My main focus is dealing with the underlying virtual hardware and OS - the forum software is updated by someone else, and Plausible was installed and then promptly forgotten about 3 years ago by yet another person. While doing som...