You booked Premium via the Apple Appstore and switched to an Android phone in the meantime or vice versa? Then you have to proceed a little differently than usual to cancel Premium.
1. from Android to Apple:
Log in to your Google account online at https://play.google.com/. Go to the profile icon on the top right and then to => "Payments and subscriptions". There you can cancel your subscription.
2. from Apple to Android:
For Apple you have to either download iTunes for Windows (https://support.apple.com/de-de/HT210384) or use Apple's phone support. Unfortunately, there is no easier way.