If you need to change the address of something you've already purchased- just make to do that quickly. You'll only be able to update the address before the seller ships. To change your address, tap ‘Me’ at the bottom of your app then ‘Purchases & Sales’. From there, you should be able to update your shipping address directly on the purchase if the seller has not printed the label yet. If the seller has printed the label, please reach out to customer support. 

You can also update your shipping address directly from the checkout page when you're purchasing something or by tapping on ‘Me’ at the bottom of your app and going to ‘Account & Settings’.