You need to manage your Apple Appstore subscriptions directly via the Appstore.
To cancel a subscription on desktop computers we suggest to follow this subscription: https://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/manageSubscriptions.
You are redirected to the iTunes Store where you get an overview of your subscriptions. You can edit and cancel your subscriptions there.
As an alternative you can cancel the subscription on iPad or iPhone. There are some more steps required. How to is described here: https://support.apple.com/HT202039.