You can try un-claiming / re-claiming the server (removing it, then re-adding it to your Plex account): You need to reclaim your server, but don't dare to touch the Windows Registry
To perform a full uninstall, see Plex Docs → Uninstall Plex Media Server.
That will completely remove Plex Media Server from your system.
You will have to remove some registry entries and a directory in %LOCALAPPDATA% (C:\Users\user_name\AppData\Local).
