Step 1: While logged in, select "Account Management" from the "Your Account" menu in the top-right corner:
Step 2: On the next page, click on the "Stop Auto-renewal" button:
Step 3 - Confirm that you want to cancel your subscription
Once you have done this, your subscription will not renew - and you can continue to enjoy the benefits of your subscription until your renewal date (when it will expire).
If you would like to come back and reactivate your subscription at any time, just login, go to your Account Management page and select the Resubscribe button.
If you are still unable to cancel (or are having any other problems) please email support.