I bought two Nook HD+ because of the low price and high screen resolution about 2 years ago.  I originally operated each Nook with Kit Kat OS off a micro sd card as I didn't want to erase the factory installed OS.  I came to find out Lollipop was available for download.  I paid $15 for the download but I was able to use it on both Nooks.  At $7.5 per upgrade, the money was very well spent.  The factory OS was wait too restrictive and slow.  Lollipop is much better when it comes to manage processor with slow speed and makes the Nook operating much more smooth and faster.  It also opened up to all the apps that are available in Google Play which it was not the case with the factory OS.