To add a new card, tap on the menu icon ☰ (top right) in app > select Payment > select Add (top left) and follow the prompts. Remember to set it as your default card after adding. To do this simply select the new card and then select the default option.
To update your existing card details, simply add a new card same as above. If the card has the same number, it will automatically override your old card with the new expiry.
You can also add/update a card during checkout via the 'Payment Method' section in your cart.