The myTracks premium subscription is fully managed by Apple's App Store. If you want to cancel the subscription, you can do it through your iCloud account on iPhone or iPad. To do this, open the iOS Settings app and tap on your iCloud account. There you will find the entry Subscriptions. Tap on this entry and then the list of all subscriptions will appear. The myTracks premium subscription is also listed here. Tap on the entry to see more details. There you can, among other things, cancel the subscription.