Easy! 

1. Go to Profile > Settings in your StubHub account
2. Under Payment options for buying tickets, click Add a new payment option.
3.  Complete the required fields to add the card to your account.

Do you want to add the same card to a different account? 

You can add the same credit or debit card to up to 3 StubHub accounts. 
You can edit your payment info to remove a credit or debit card from one account.