The instructions here are specifically for changing the information on your Premium Service subscriptions. If you are trying to update the billing information for your custom Domain name, you can manage the information through the Domains section of premium services. Click here for step-by-step instructions.
From the Webs Premium tab of your Dashboard, locate and click the Billing & Subscriptions link at the bottom.
This will launch the Billing Information page.
From the Subscriptions panel, click the Manage link next to a subscription you want to manage. In this case you are choosing your Premium package.
This will launch the Subscription Management popover.
Note: If you see "Legacy Account" in the Renewal Details section you will need to cancel the existing subscription before you can add a new payment method for the subscription. Instructions are included in the Cancel this Subscription link in the Manage popover.