Removing Payments is only possible through the web portal. This is currently not possible on the mobile app.
To remove a payment from your client portal account, you will have to log in to your account and select the "Invoices" tab. You will want to select the "Payment Methods" button. This will take you to a screen that will initially show you the option to add a new card. You can scroll down on the page to see the option to delete any payment you have added.