You can do with via the Afterpay website or in the Afterpay app
Website
Log in to your account via www.afterpay.com
Choose - My Account
Click on - Billing (Icon is a card)
Click – Add Payment Method OR Remove Card Afterpay App
Click on Profile - > Payment methods - > Add a new debit or credit card
Please note that you won't be able to remove the preferred card on your account or a card that has associated orders, so if you are trying to remove your current 'preferred card', change this first as well as updating your orders then you will be able to remove the card.
Good to know
When adding a new card to your account, Afterpay will verify it by issuing a temporary small authorisation hold of $3 or less. Authorisation holds are small amounts of money that are never actually charged to your account and should be voided from your payment method within a few business days.
Refer to here for more information on changing your card for existing orders.