If you started a Premium trial via Google, you can cancel your trial through Google Play at any time by following these steps:
1. On your Android phone or tablet, open the Google Play Store
2. Tap Menu --> Subscriptions
3. Tap TinyTap
4. Tap Cancel subscription
5. Follow the onscreen instructions
*instructions provided by Google (https://support.google.com/googleplay/answer/7018481?co=GENIE.Platform%3DAndroid&hl=en)*