To manage your payment methods and history, go to Your Account by clicking on the white arrow on the top right corner of your screen.
From your Dashboard you'll find this option on the left hand side, below your profile info. Once on Your Account, select the “Plan” tab and click on Add Card in order to manage your billing information.
On this same tab is where you manage your subscription. Here you can upgrade your plan, downgrade it or even terminate the service.