To manage the purchases and subscriptions you've made, go to the "Your Purchases" menu on your profile.
How to Unsubscribe from a Subscription Plan
You'll find the option to unsubscribe from automatic renewals under Your Purchases. If you purchased on iOS, you'll find the option to unsubscribe within the iOS app.
Here's what your purchases will look like on iOS:
This is how the detailed view looks if you're logged in on the web, or in the Android app:
For any subscriptions you purchased on the web or Android, you'll see a "Cancel Subscription" button.
For subscriptions you purchased via the iOS app, you'll find a handy link that will take you to the appropriate spot to cancel your subscription in iTunes with your Apple ID. Please note that this button only appears for recurring monthly or annual fees.