Cancel subscription? No such thing, it's a one-off purchase

Subscriptions in the App Store can be confusing. To make it worse, Apple sometimes waits for several days before charging you after an app purchase, so it can look like you’ve been charged more than once.

It is an old-school “paid app”: you can buy the app, and it is a lifetime, refundable, one-off purchase. Once you have purchased it is yours to use forever and you will never be charged again for it.

Check your App Store purchases

If you are not sure you can check your App Store purchases. To see how, here is a link to Apple’s instructions: https://support.apple.com/HT204088