How to manually update your Galaxy Nexus (takju) to Android 4.2

Be warned though, updating manually does have some pretty strict requirements right now. Unlike the Nexus 7, the only way to manually push the update is via ADB, so you have to have the Android SDK installed, you have to be comfortable in the command line, and you cannot have any of the 4.2 apps flashed on your device already. That last one is key. If you've flashed the new keyboard, camera app, or anything else, you'll have to flash back to stock Android 4.1.2 build JZO54K before proceeding, because the install will fail.

So, first off, you must be running stock JZO54K to run the update, and as always you are responsible for any problems you may encounter. 

1) Download the JOP40C update zip file

Reboot your Galaxy Nexus, holding both the Volume Up and Volume Down buttons while it boots up. When you get to the fastboot menu (it says "Start") press Volume Up until it reads "Recovery" then press the power button. The device will switch to Recovery mode, and when you see an Android with a red exclamation point icon, press Volume Up and the Power buttons together, which will bring up the Recovery menu. Select "apply update from adb."

2) Make sure you have adb (if you don't, download the Android SDK and install the updates). Connect your Galaxy Nexus to your computer with a USB cable.

3) Open up a command prompt (cmd in Windows, or terminal in Linux and Mac) and navigate to the same directory as the JOP40C zip file you've downloaded, then type in the following:

adb sideload e587de13bf8a.signed-takju-JOP40C-from-JZO54K.e587de13.zip