Updated for 7.0
Click on the Settings icon (the gear) located to the right of the search bar. This will take you to this page:
In the Accounting box, select Credit Cards. This will take you to this page:
To edit an existing credit card type, click on the name of the credit card type.
Rename the credit card type.
Change the fee to the right of the name if necessary.
To make a credit card type inactive, click the button to the right of the fee that says ON.
When done, click the Update button in the bottom right-hand corner to save changes.