You can cancel the recurring payment for your Plan in the Settings on your iOS device:

1. Go to Settings > [your name] > iTunes & App Store.
2. Tap your Apple ID at the top of the screen, then tap View Apple ID. You might need to sign in with your Apple ID.
3. Scroll to Subscriptions, then tap it.
4. Tap iStudiez Pro Subscription. 
5. Use the options to manage your subscription. 
6. You can choose a different subscription offering, or tap Cancel Subscription to cancel your subscription.
   If you cancel, your subscription will stop at the end of the current billing cycle.

Please follow this link to read more:

https://support.apple.com/HT202039