If you are the account owner, then you can change the billing details within your account dashboard.  If you are not the account owner, you will not be able to make billing changes to your account.  Please contact the account owner or transfer ownership.

As the account owner, follow these steps:

1. Login

Click the login link in the top right of the header navigation to open up the login dialog box and enter your login details

2. Click the Billing and upgrades link in your account navigation

3. Click the "Modify your payment method" link under Change payment method