Once you are signed in, Click on “More” on the bottom right of your app toolbar. From there, you can update your account info, addresses, payment methods, notification preferences, and view your won items.  

To add a new payment method, make sure to complete all fields without any errors and that updated information will be saved to your account.  You can choose to make any card your primary (the card that will be charged first) as well as delete any old or no longer needed payment methods.