To update your documents once you register for the first time, or one of your documents was rejected or expired, you can head to the "More" section of the App and into the "Documents" area.
Here you will then be able to upload for the first time, or else substitute any rejected/expired documents.
To see any the currently uploaded documents, simply click on the "View" widget next to each document, while to upload for the first time and/or substitute any document click on the "Upload New Document" button, which will then allow you to select uploading an image from your gallery or else take a new picture of the document.