POP subscriptions are only available on the web, and not in the app stores, so the payment gateway will be either PayPal or PayPro Global:
– How do I Cancel my Subscription in PayPal?
– How do I Cancel my Subscription in PayPro Global?
Inside each gateway, the cancellation will take effect at the end of the current period.
If you experience any problems, contact the SimDif team via the Help center in the app. Look for the '?' icon in the bottom left corner, and go to the 'Assistance' tab.