Via your customer portal account click the Payment Methodssub menu, on this page you will find a button that reads Add new payment method, use this button to add a new card.
Once the new card has been added click the Shipping Addressessub menu, on this page click the drop down arrow and then click change payment method.
Once your subscription has been moved to the new payment method you can remove your old card via the Payment Methods sub menu.
If you are having issues with any of the fields, please try using a different browser or device as a browser caching issue may be occurring. If the issue persists, please reach out to our customer support team for further assistance.