I signed up for a free trial of STARZ. I do not want to extend this service any longer. Can't figure out how to cancel and not have the monthly fee charged to my Apple TV Plus account. Can you help me with this?

How to manage your subscriptions: See or cancel your subscriptions - Apple Support

For clarity. Stars would be billed to your iTunes account. Apple TV Plus is Apple's streaming service. You can follow the link above to cancel, which will turn off auto renewal for any future payments. If you have already started a new billing period then it will not refund any amount that has already gone through.