If you need to cancel your subscription for on iOS, follow the steps below to have your account removed.
If you would like to cancel your premium subscription to FitSW, follow this guide by apple:
https://support.apple.com/en-us/HT202039
1) Finding your phone settings app.
2) Click on your Apple ID name to bring up your settings associated with your Apple ID.
3) Click on the Subscriptions option to show all of the apps that Apple is managing subscriptions for.
4) Choose on the active subscription for FItSW and navigate to the bottom of the landing page.
5) Hit cancel subscription and then you will have successfully ended your subscription with FitSW.