To add a card: My Balance > Manage Payments > Add/Remove Cards > Add New Card > Enter Card Details > Save Card
To remove a card: My Balance > Manage Payments > Add/Remove Cards > Select and remove the card you wish to remove > Tap on the three small dots on your selected card > Delete Card
PS: You can also update a card while adding cash. There is no limit to the number of cards you can save in your Dream11 account.