These are the forms that allow customers to update the shipping address after checkout when they proceed with addresses that cannot be validated. They are accessible via the Thank You page after checkout or on the email notifications.
You can customize the color and other parameters with your custom CSS, and you can also change the verbiage to your liking!
App dashboard > Update forms
Please note, if you're a Shopify Plus Store and do not want your customer to be able to update their address after checkout you can do that here:
App dashboard > Address prompts > Advanced options > Do you want to disable address validation after checkout? > Yes