Because Apple manages all subscriptions purchased through the App Store, you'll need to follow these instructions on your iPhone in order to cancel your free trial:
Go to Settings on your iPhone
Tap Your Name at the top of the screen
Find the Dipsea subscription
Tap Cancel Subscription. If you don’t see Cancel Subscription, the subscription is already canceled and won't renew
Note: A free trial must be canceled before the trial's end in order to avoid a charge from Apple (iPhone) or Google (Android). If you've been charged and would like to request a refund, you can find instructions for requesting a refund from Apple here and from Google here
To learn how to cancel a free trial on an Android device, click here.