To add or edit a customer's credit card on file, first navigate to the Customers tab.
From the Customers tab, find the customer you want to add a card on file to. Click on the name of the customer to open the customer's profile.
When you are on the customer's profile, scroll down until you see the "Credit Cards/ACH" section on the right-hand side of the screen:
Click the Add a Credit Card button. You will then see a popup to add a card on file:
Enter the card information and click the Save button. Your selected customer will now have a card saved on file.