I just started having the same problem. I tried deregistering and registering again per the tech people and it did not work. I powered down and powered up numerous times and it did not work. I am under warranty so they are sending me another one which sucks from the standpoint that I have lost all of my achievements on all of the angry birds games and cut the rope game. Everything else on my nook works great. The kicker is the one they send me is either a new one or certified pre-owned. Who heard of a warranty replacement being a certified pre-owned.
Not sure if this will work but it's worth a shot.
At the top right of your Apps Page is a link 'Check for Updates'. Click on that.
Wait a minute or two. Then a popup with available updates will show up.
Then you can update.
I make apps and I'm not aware of any way to have an 'update' option from within the app. I only know updating via the Apps page.
I have the answer to my particular problem (updates of apps getting stuck in download loops).
After reading some other posts here I realized it was a memory problem. I archived all apps I wasn't currently running ( I do wish they wouldn let me get rid of Chess and apps that came loaded), and I was able to then successfully update angry birds rio and others that kept asking to update over and over, like my magazines.