It’s easy to find out. Navigate to Thunderbird Beta in F-Droid Basic and look at Versions. If you only have a single version there, then you have to delete it. If it says that no version with a compatible signature was found, so too. If you have three versions to choose from and one of them is installed, then you don’t need to do anything, because then you will get updates.
Looks right. Then you don’t have to do anything. I mean, it’s up to you, but do you really want to continue using the beta?