Once on the Billing page you have the option to select +Add payment method and will prompt you to enter in new card details.

If you do not have the +Add payment method check invoice history if one is past due, you will need to pay the past invoice before the +Add payment method appears

If you do not have the Billing tab available - select the intercom chat box at the bottom of your screen or email support@pulley.com and support will have the billing page appear on your account.