Just after users began reporting that their Galaxy Nexus devices were receiving an update to Android 4.2 Jelly Bean (build JOP40C, to be precise), it looks like the manual update package is available for download. As could be expected, this coincides almost perfectly with the launch of Google's newest additions to the Nexus family – the Nexus 4 and 10.
Of course, it's worth reiterating that this is an update specifically for the Galaxy Nexus' Takju variant – that means only Galaxy Nexus handsets bought from the Play Store need apply (we're still waiting to hear about the Yakju variant). Your device should also be running build JZO54K before trying to install the update manually.
For those ready to download and install manually, the process is easy. If you have a custom recovery, just download the zip (linked below), move it to your device, and flash as usual. Those with stock recovery should keep the file on their computer, and just follow a few easy instructions:
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 e587de13bf8a.signed-takju-JOP40C-from-JZO54K.e587de13.zip
You should see a progress bar, which will eventually hit 100%. At this point, your device is installing the update.
Reboot and enjoy! You are now running Android 4.2 on your Takju Galaxy Nexus.
Download: Takju Build JOP40C (md5 06d0d82520c8ab43124c63f3bb147f3d)