I’m writing this presuming you’re local (same subnet) to the server.
If it’s remote then you’ll need to ssh tunnel and use the loopback address to claim it again. Otherwise:
Settings - Server - General
You’ll see the “SIGN OUT”
Sign it out
Sign out of Plex/web (upper right)
Sign in to the account you wish to use
CLAIM the server
Once claimed, Restart the server so it downloads the features of the new account.