If you signed up on the Desktop Site, you must cancel your subscription on the Desktop Site. If you signed up on the Apple iPhone app, you must cancel your subscription through Apple and unfortunately we cannot intervene with their billing process.
How to find out where you signed up:
On the iPhone & iPad app:
Tap on your Profile in the bottom menu bar
Tap on your Collection in the middle of the screen
Tap on "Subscription" and it will tell you how to cancel
On the Desktop Site:
Click your profile icon in the top right corner
Select "Account"
Toggle to "Billing" in the menu bar below the profile icon
Click "Update billing information"
On the right side of the new window, click "Cancel subscription"
On the App Store:
Open the Settings app
Tap your name
Tap Subscriptions
Tap the subscription
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.
If you need any more information, please see Apple's help article here.