You can find your Apple subscription management inside your account settings for your Apple ID, which can be accessed for example via the App Store. There you can can cancel any subscription and trial subscriptions.
- Open the Settings icon on your iPhone or iPad and tap on your iTunes account or name in the upper part of the screen
- Choose iTunes & App Store
- Tap Apple ID:
- Tap View Apple ID and Sign in with your iTunes password
- Scroll down and tap on Subscriptions
- Tap Cancel Subscription (you will receive a pop-up message to confirm cancellation).
Once all is done, you are all set ! This means that no other charge will happen and your subscription has been stopped.
If you want to have a payment examined, please visit https://reportaproblem.apple.com and explain, why you want a refund. It's usually a fast procedure.