1. On a machine (not phone) and click on balance.
  2. This brings up the account History page. 
  3. Click the blue "Add Money" button and you're taken to a new screen where you enter the amount. NOTE A: do not attempt to change your card by clicking on the "Saved Credit Cards" button. Follow this process instead. 
  4. Click the blue Add Money button again and you'll get another popup asking for your card number, month and year (enter the four digits in a row, no slash) and then the CVC. 

NOTE B: If the credit card info showing in this popup window is your old credit card info, tap or tab or use your mouse to put the cursor into the credit card number field and hit the delete or backspace key to erase all the information there. Enter your NEW information.
 5. Then click the blue "Pay $xx" button at the bottom.

NOTE C: Click the "Remember me" button and your new card info will override your old saved card.