How do I cancel Apple TV?

You'll need to log in to the Apple TV website from a computer, Mac or PC, with the Apple ID being used on the Fire TV, and cancel the subscription there.

See here: https://www.pcmag.com/how-to/how-to-cancel-your-apple-tv-plus-subscription

https://support.apple.com/billing

You can’t subscribe on your Fire TV, so I suppose you will have to cancel on a different device as well.

See or cancel your subscriptions - Apple Support