To update your payment method(s) in the app, open the app menu using the ☰ button, tap your name/profile picture, and then scroll down to the Connected Cards section. Here you can add new payment methods. Select an existing payment method to see the edit and delete options.
To update your payment method(s) on the web, tap your profile picture in the top right corner of the screen, select Profile, and then open the Payment section. Add a new payment method, or select an existing one to edit or delete it.
When adding a new payment method our payment processor may use a small transaction (usually $0 or $1) to verify that the card is valid and the bank will allow it to be authorised. This temporary transaction will either be refunded or completely removed from your list of transactions within a few days (the timing of this is up to the card-issuing bank).