You can do this online after you login.  Click on the "Your Account" link in the upper right corner of the Members page to get started.

1. First, click on the "Upgrade" button for the "Extra", "ExtraRT" or "PRO"  service. 
2. Click on the "Click here" link where it says "Click here if you would also like to add time to your subscription."
3. Select the number of months you want to extend.
4. Click "Continue" to proceed with the order.