Step 1: While logged in, select "Manage Account" from the "Your Account" menu in the top-right corner:

Step 2: You will see the following screen: 

Step 3 - Select "Cancel Subscription" as shown and you will then see the following scree asking you to confirm cancellation 

Once you have confirmed, your subscription will not renew - and you can continue to enjoy the benefits of your subscription until your renewal date (when it will expire).

Once cancelled you will receive confirmation via email as well the following message

If you are still unable to cancel (or are having any other problems) please email support.