How to cancel an app. Just ordered

You just delete it from your phone. If it has a subscription you need to cancel the subscription. You can go to this site, cancel the subscription, and request a refund→Subscriptions and Billing - Official Apple Support

If you have already downloaded the app then you can certainly deleted it. If you have paid for it then use the app itself to cancel the subscription (in app's settings), if available. If you paid for it and need to cancel payment or request a refund then start here ➔ Refunds, Subscriptions & Billing Questions

Axel F.