If you need to update your payment information, go to Screambox.com and log-in to your account. Once you are logged in, click your name on the upper right of the homepage. Then click "Your Account" in the drop down list.
You will be taken to a screen with your account information on it, contained in four boxes. Click on the "Update Payment Info" link in the Subscription box on the top right.
Now you will see an "Update Payment Information" box which will allow you to update and change your payment information for your subscription.
Make sure to save your changes and you will be all set!