Q: How to cancel App purchased from iPad

Cancel what exactly?

Apps are a one time purchase.  

If you require a refund for a purchased App, you can go to http://reportaproblem.apple.com and report the issue with the App and request a refund.

If you are subscribed to a monthly service on an App and whish to cancel it you need to set its auto renewal to off by following this support document:

View, change, or cancel your subscriptions - Apple Support

All sales are final.

As provided, you can try asking for a refund.  Apple is under no obligation to do so.