can i change my subscription from Paramount Tv Showtime to just Paramount TV

Your welcome. If you feel your question has been answered please mark the thread as answered. Doing so may help another user.

Did you subscribe through the Apple TV app or did you subscribe directly to Paramount and to Showtime?

You can however if you got the bundle price of having them together you would not be able to get Paramount + for ½ of what the bundle costs. IMHO, leave it alone and save a few $$ per month.