Just a heads up, if you cancel only partway through a yearly subscription, we are currently unable to offer a refund for the remaining time! Sorry about that.
To cancel your subscription:
On an iPhone:
1. Go to Settings > iTunes & App Store
2. Tap your Apple ID at the top of the screen
3. Tap View Apple ID. You might need to sign in or use Touch ID
4. Under Subscriptions, tap "Manage"
5. Tap the subscription that you want to manage
6. Tap Cancel Subscription to cancel your subscription. If you cancel, your subscription will stop at the end of the current billing cycle
On an Android:
1. Open the Google Play Store app
2. In the side menu, open Account menu
3. Tap on Subscriptions
6. Tap Cancel to cancel your subscription. If you cancel, your subscription will stop at the end of the current billing cycle
Here is an alternative explanation from Google : https://support.google.com/googleplay/answer/7018481