Thanks. It's good to know this.
When you "add new item", the epub file is placed in the "My Barnes & Noble eBooks" folder in your "My Documents" folder of your computer. Whenever you update your epub (which I do a lot when I'm formatting), just close the Nook software and replace the epub in that folder with your new one. The next time you open your Nook software, it will use the new version.
This saves time.
Kind regards, David