To set your payment method in your name cheap account, you will first need to login to your Namecheap account.
To do so, go to Namecheap.com, then move your mouse over the sign in button in the top-left corner.
You should see a dropdown with a login form, prompting for a username and password. Enter the username and password you have, then click the Sign In button.
Once logged in, click on the Profile tab at the bottom of the left sidebar.
Once the Profile page is loaded, click on the Manage button in the Payment section.
Click on the Add New Card button.
You will see a form to enter your card details in, along with your billing information. When you are all set, click the Save Changes button at the bottom.
At this point, you should be all set.