You can update your card anytime via the Settings tab of your Virtual Mailbox.
To update a credit card:
Log In to your account, navigate to the Settings tab and then Subscription & Billing
Subscription & Billing > Plan & Billing Details
Click on Plan & Billing Details and scroll down to click on View Payment methods
3a. To update an existing payment method, click on the credit card:
3b. Click "Edit Payment method"
3c. Click "Edit Payment method"
4. Set this card as "Primary" to use for all your automatic payments. Or as "Backup" to use only when the primary payment method fails.
Your credit card should now be ready for use.
5. Similarly if you want to add a new card just click on Add new and fill the details.