Go to the Order tab in the bottom row of tabs on the contact record
Click on the name of a subscription to open it
Go to the Payment Plan section and click the Edit Payment Plan button. Select the new credit card from the drop-downs or click on the Add button to enter a new credit card.
Click Save to update the subscription. The past due billing will be handled in one of the following ways:
If the card has not reached the maximum number of autocharge retries, the next autocharge attempt will run all current and past due charge.
If the maximum number of auto-charge retries has been reached, you need to manually run the past due charges. All future charges will run automatically using the new card.