You go to the App Store (logical place to look), tap your profile pic (a standard way of communicating "my account"), and tap your name in the menu. You'll arrive at the Account Settings page which has an obviously labeled Subscriptions button. Once there, subscriptions are neatly sorted into active and expired. Clicking on a subscription instantly shows you all options, you can change among them and get a full prorated refund for time on the old plan. You can cancel with ONE tap (and a confirmation)--cancelling does not interfere with what you've already paid for, and it tells you exactly what date you've paid through. All auth is by Touch/Face ID.

I've never seen a simpler and more straightforward billing UX as Apple's.