There's two ways to do it. You can either replace the operating system (called the ROM) directly or you can run a new ROM from an SD card. Running it from an SD card leaves the Nook intact if you ever need to take to a B&N store or return it. I've been running mine from an SD card, but had a couple issues (see my post in the Nook Developer section). If you want to give it a try,
here's the link. Do a Google search for directions on replacing the ROM.