Setting>iTunes App Store> click on your Apple ID & View Apple ID > scroll to subscriptions > select subscriptions and then you should be able to click cancel subscription. Make sure you do it before the 3rd day ends.
Here's how to cancel on iPhones.
Tap on your Apple ID.
Tap View Apple ID when the pop up window appears.
Enter your Apple ID password or fingerprint ID when prompted.
Tap the Subscription you want to cancel.
Tap Cancel Subscription.
Tap Confirm when prompted to confirm that you want to cancel your subscription.