Please follow the instructions below:
Steps are shown below as it would show on iOS device:
1. Access Settings in the home of your device
2. Scroll down and tap 'iTunes Store and AppStore'
3. Tap your e-mail and Apple ID
4. Click 'View Apple ID'
5. Scroll down and tap Subscriptions
6. The trial will already be highlighted - click 'Cancel Trial'
7. Then click 'Confirm'
8. The trial will then not be highlighted and unticked - meaning it's cancelled