In case there is any confusion for future readers, the link to update.zip for version 8.1.7.120 is:
http://su.barnesandnoble.com/nook/pi...120/update.zip
Also be sure to follow the instructions completely, if for some reason you get stuck in a boot loop after flashing a bad boot.img, holding all the physical buttons (power and all four page turn buttons) then plugging in the usb will start fastboot mode.