How can I disconnect UpdraftPlus from my Microsoft OneDrive?

You can do that from your this page in your Microsoft account.

Please note that Microsoft only provide a facility to disconnect UpdraftPlus completely – i.e. all instances. They do not provide a way to disconnect only one website (if you happen to have multiple websites that are backing up to OneDrive). To disconnect websites individually, you will need to log in to the WordPress dashboard and deselect OneDrive as a backup option.