You can cancel your Premium subscription auto-renewal by navigating to your Account Management page, clicking "Manage Subscription", and then choosing "Cancel subscription" on the next page. Account Management is only accessible from an internet browser and isn't available on the mobile app.
Once you cancel your subscription it will remain in Premium until your expire date. Your account will then downgrade to Basic. You can continue to use your account after that, but will not have access to the Premium features.