How to cancel my AppleCare monthly payment for my iPhone?

These are the instructions:--> https://support.apple.com/en-us/HT202704

Regards

Giulio

Visit Settings, General and then About, you'll see the plan Subscription there. If you purchased an annual plan, you will need to contact Apple Support or will calculate the refund based on the months you have left. If you have a monthly plan, then simply cancel the subscription in the normal settings.