For Web: To add a credit card on the web, log into your account, click on the top right where it says "Hi, ____" > My Account. Below your profile and addresses, there will be a “Payment Methods” section. From here, click Add New +.
For iOS App: Log into your account, click the "Account" tab at the bottom > Settings > Billing > Add New Credit Card
For Android App: Log into your account, click on the three bars on the top left for the main menu and go to account / billing