As @napolij mentions, on Windows, Plex stores the information in the Windows registry.
You need to claim the server with the desired account. An uninstall/re-install will not do so, as it does not remove the necessary Windows registry settings.
To remove the current owner from the server (unclaim it):
See You need to reclaim your server, but don’t dare to touch the Windows Registry .
Alternately, if you are comfortable manually editing the Windows registry, see Why am I locked out of Server Settings and how do I get in?.
After unclaiming, reboot the PC.
Then claim it with your account: New Claiming Requirement for Windows.
