Your payment method for Sling will be handled on MyBundle (if you are subscribed to Sling through MyBundle.)
To change your payment method, if you subscribed through MyBundle, you can log in to your MyBundle account and visit your profile settings page. Select “Credit card info” under the “Profile Details” section and update your card information as needed. Make sure to select the "Default" card that you want to be charged (if you have more than one card on file).