I've updated several hosted installations to v20 without issue. I also have one on-premise 3CX install, that currently sits at 18 Update 9 (Build 31). Where can I find the procedure to update this guy to V20?
For the Windows installation there is no upgrade path. This is an option only for the Debian installations. For Windows you will need to create a full backup and re-install with the V20 Windows installer. Keep in mind that at the moment the V20 Windows installer is in the V20 Update2 BETA version and can be found here.