How do I change a credit card on an existing order?

To change the credit card on an existing order, open the Purchase Order review page. 

Scroll down to the Payment Terms section of the order review and click +Add Payment Method.

This will open up a pop-up where you will enter the credit card details. Click Add Payment. Anytime you add a new credit card to an order, the card details will be saved to the account and will appear on the list of available credit cards for use on all the account's future orders. 

Select the new credit card by checking the circle next to is. This will add it to the order and allow the vendor to charge it.