Hi jonks -
Nortel's website has the code images you can download, along with the release notes that spell it out fairly well.
Be advised you have to have a current support contract to be able to download the images from their site - I use (2) 8100s, and my contracts only let me download 8100 images, for example.
It boils down to three main files - a boot monitor image that you burn to flash (consider it the BIOS of a PC); a runtime image (consider it the OS); and the ethernet code. There's also an ATM image for ATM cards; a WSM image for the Alteon module; and an encryption module of some sort for SSH if you turn it on. If there's others I've missed, someone please correct me.
You'll be instructed to copy the images to internal flash and/or PCMCIA flash, followed by booting with the boot monitor code to upgrade the onboard PROM, followed by booting again with the new runtime image. You'll also have to update the boot.cfg files using the config bootconfig commands; and, re-save (and verify) your config.cfg file to upgrade the config.cfg to a version compatible with v3.1.2
That's probably too much info... let us know how you progress, and someone should be able to answer your next question(s)...