Under the Billing section you will find details of the account plan. If you click the Change link it will allow you to enter a new credit card.
Your Subscription: Type of billing
Messages Used: All messages sent over all campaigns
Total Contacts: Total active subscribers over all campaigns
Billing Info: Click Change to update credit card information
Next Cycle Date: Next time recurring monthly charge will be billed
Credit Card: The credit card on file that will be billed