Summarize this article:

Hold the Nook upside down and locate the micro USB port, which is located next to the speakers and the audio jack. It looks like a rectangular hole that you plug a cord into.It is about inch (.6 cm) wide. Hold the Nook securely with the screen against your non-dominant hand. Place the palm of your dominant hand in the middle of the back of the Nook. Use heel of your hand to create leverage when you pry the back off. You'll find a small notch on the bottom of the back cover, right by the USB port. Slide the fingernail of your middle finger over that notch. Place your other fingernails in line with the notch along the bottom edge of the Nook. Gently pull at the notch with your middle finger and press against the back with the palm of your hand. As the cover starts to come loose, slide your fingers down along the edge to remove it completely. Pull the cover off slowly so it does not bend or break.

Summary:
Locate the micro USB port. Place your palm against the back of the nook. Hook the notch over the USB port with your fingernail. Pry the back cover off.