Disclaimer: Android Police isn't responsible for any harm to your device - proceed at your own risk.
Reboot your Galaxy Nexus, holding the Volume Up and Volume Down buttons while it boots up. Once you see the fastboot menu and "Start," press Volume Up to navigate to "Recovery" and press the power button. Once you see an Android with a red exclamation point icon, press Volume Up and the Power button together, which will bring you to a new menu. Navigate to "apply update from adb."
Connect your Galaxy Nexus to your computer with a USB cable.
Make sure you have a recent version of adb that supports the sideload command (if you don't, download the latest Android SDK to ensure your adb is up-to-date).
Open up a command prompt (cmd in Windows, regular shell in Linux and Mac) in the same directory as the JOP40C zip file you've downloaded.
Type in the following:
adb sideload fdf06e8d71c7.signed-yakju-JOP40C-from-JZO54K.fdf06e8d.zip
Reboot and enjoy! You are now running Android 4.2 on your Yakju Galaxy Nexus.
Download: Yakju Build JOP40C