If you have a credit card on file, it will be the default option presented to pay for registrations.
If you need to update the credit card, you can do so on the checkout screen. The credit card presented to you is a dropdown field, and one of the options is to add a new card.
If you want to update your credit card outside a registration event, you can do so from your Account Settings as described here: Adding and editing credit cards on your profile for automatic payments.