Deleting your stored credit card details is only possible on the website, not on the app. You may delete them via Profile > Account settings, and scrolling to the bottom of the page where you will see a box for credit card. If you have previously indicated that you would like to save your credit card details when making a booking, there will be a button on the right-hand corner which states 'delete card'. Simply click on that button and your details will automatically be deleted.
If the 'delete card' button is not appearing and there are no credit card details shown, it is likely that you did not opt to save your credit card details previously.