In order to add a new credit/debit card to your account you will need to have an unpaid invoice, this could be the next renewal invoice of your account.
Once you have it, follow these steps in order to pay the invoice:
Log in to your account: https://billing.stablehost.com/clientarea.php
Go to Billing -> My Invoices and find your due invoice and click on the invoice number.
A new browser window will open with your invoice.
Click on the combo box that is below the word UNPAID.
Choose Credit Card as Payment Method.
Click on the “Pay Now” button.
Continue with the instructions that belong to the chosen payment method.
Now you've paid for this time.
Follow this if you have multiple credit/debit cards on file.
Make sure the same Credit/Debit card you used this time will be used in future times
Log in to your account: https://billing.stablehost.com/clientarea.php
Go to Billing -> Payment Methods
Click on "Set as Default" on the Credit/Debit card you used this time.
Make sure that Credit Card is your default payment method and it will be used in future times
Log in to your account: https://billing.stablehost.com/clientarea.php
Click on “Hello <Your Name>!”
Click on “Edit Account Details”
On Payment Method, choose Credit Card.
Click on “Save Changes”.