Updating digital products has been one of the key issues that people are concerned with for buying digital codices and other Games Workshop products. This article appeared on the GW website directly to help customers update their products rather than deleting and re-downloading.
Thanks to Andrew for pointing this out to me for the site. I am sure it will help others that have updating concerns.
via Games Workshop
Update to iBooks 3
Make the most of Games Workshop's digital books by updating your iPad to the latest version of iBooks.
With iBooks 3, you'll be notified as soon as one of Games Workshop's fantastic digital books receives an update. This could be a FAQ update to a Codex or Army Book, or the latest month's addition to "A Call to Arms: Ultimate". Once you've been notified of the update you can simply update the book, rather than having to delete it and re-download it. It saves you time, keeps you up to date, and now you won't lose any notes or bookmarks that you may have added.
To update to iBooks 3, you first need to make sure your iPad's iOS is updated to 5.1 or later. To update your iOS, access your iPad's "Settings", then select "General". Now choose "Software Updates". You'll then be able to update to the latest version of iOS.
Once your iPad is updated to the correct version of iOS, to update iBooks you have to access the "App Store" app. Then choose the "Updates" before selecting iBooks.