I want to cancel my apple tv subscription.

See instructions in If you want to cancel a subscription from Apple - Apple Support including the following:

Tap Cancel Subscription. You might need to scroll down to find the Cancel Subscription button. If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.

and if needed, see the "Can’t find the subscription that you want to cancel?" section of that support article and if nothing else is working: Contact Apple Support

Another option is to use an internet browser as described in How to cancel Apple TV+ - Apple Support