On Android devices currently you can only change card info while you are placing an order. If you create a card and proceed to the final screen just before checkout you'll see a link "Payment method". Taping on this will load a screen which will allow you to either scan a new card using your camera, OR you can click the link in the bottom corner "Enter Manually".
On iOS devices you can swipe in from the left edge of the app's first screen to get the main menu. From there tap on "Account" near the bottom of the menu. You'll see your current card info among other things. If you tap on that card info you'll get a few choices to either remove or to update the card info.
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