Mr. Chromebox is a generous developer. Go to the page for his Chrome OS firmware script and type out the command on that page (starting with cd; curl ...

Choose "Install/Update Full ROM Firmware" from the menu by entering "3" and hitting Enter. You'll be prompted to agree to the inherent risk of what you're doing a few times by pressing "Y" for yes.

You'll also be prompted to create a backup copy of your stock firmware. You should answer "Y" to this, and grab another USB storage device, if you have one. Once you've backed it up onto a USB drive, back it up somewhere online, too!

Once you've finished installing the new firmware, enter "P" to power off.