From the Model Mayhem FAQ:

How do I update my credit card or add a new one?
To add a new card:
  1. Login to your Wallet from a computer and with either the Firefox or Chrome browsers.
  2. Locate and click on "Add New Card" underneath your current card information.
  3. Add in your new card information.
  4. Check "Make Primary" and save the changes.
  5. Delete the old card from your Wallet by choosing the Delete option shown beside the card.

Please note that if you do not set your new card as the primary card, and remove your old billing information, the system will likely attempt to bill your old, primary card information.

To update an existing card (such as changing your expiration date):
  1. Login to your Wallet from a computer and with either the Firefox or Chrome browsers.
  2. Locate and click on the “Edit” next to your existing card information.
  3. Add in your new card information. Save the changes.