You can add a new payment method in the app by tapping on the profile icon in the upper right-hand corner of the map view, and then selecting "Wallet/Pay by App". In the top right-hand corner, you will see a plus sign that will allow you to add your payment details. Once this is done, tap on the "Save" button to finish the process.
You can save multiple credit cards to your account. However, please note that existing payment method details cannot be changed (i.e. card details entered incorrectly). To update a payment method you have to delete the old payment method first. To do that, tap on the payment method in the "Wallet" section, and then tap "Remove Credit Card".