You can easily manage your subscription here!
If you want to cancel your subscription, you can also go through the following steps:
1. Access Settings on your device;
2. Scroll down and tap "iTunes & App Store";
3. Tap on your Apple ID (your email), and then on "View Apple ID" and enter your password;
4. Scroll down to “Subscriptions” and select "Math Learner";
5. Tap on “Cancel Subscription” at the bottom and confirm the cancellation!