how to cancel a subscription purchased through an app

Again, see the instructions from If you want to cancel a subscription from Apple - Apple Support

The iPhone/iPad part of those instructions are posted below:

How to cancel a subscription on your iPhone, iPad, or iPod touch

1. Open the Settings app.
2. Tap your name.
3. Tap Subscriptions.
4. 
5. 
   
6. Tap the subscription.
7. Tap Cancel Subscription. You might need to scroll down to find the Cancel Subscription button. If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.

If you can't find the subscription that you want to cancel

See instructions in If you want to cancel a subscription from Apple - Apple Support

Sorry, I don't remember where it was in iOS 12. Try going into Settings and doing a search there for Subscriptions.

or click on:

Contact Apple Support