I would like to change my subscription from Apple plus tv to just Apple TV. How can I do this?

There is no subscription to Apple TV. The only subscription is Apple TV+. If you want to cancel that, see instructions in How to cancel a subscription from Apple - Apple Support or you can use an internet browser to cancel as described in How to cancel Apple TV+ - Apple Support

The Apple TV+ subscription is a stand-alone product/service. You can cancel that, if you do not want to continue with it.

“Just Apple TV” is not something to subscribe to. You can use the Apple TV box and the Apple TV app without subscription (and thus without Apple TV+ content).