1. Download the 4.2 factory images for Takju from http://goo.gl/UMbmS
2. Extract files using 7-zip on Windows (you need to extract twice and will get the files as: bootloader-xxx.img; radio-xxx.img and image-xxx.zip)
3. Copy these files to the location android-sdk/platform-tools/ (if you have the android-sdk installed or to the location where adb & fastboot is)
4. Plug in your Galaxy Nexus (ensure Debug option is enabled under Developer Settings
5. Follow these commands:
a. adb reboot bootloader
b. fastboot flash bootloader bootloader-xxx.img
c. fastboot reboot-bootloader
d. fastboot flash radio radio-xxx.img
e. fastboot reboot-bootloader
f. fastboot update image-xxx.zip
This would update your Galaxy Nexus from Stock 4.1.2 (yakju) to Stock 4.2 (takju) without losing any data/apps
1. If you using Windows follow the video tutorial to setup adb and fastboot http://goo.gl/jGcic
2. If you are on Mac, use ./ before the commands for execution
Note 2: With this method you don't need to remove the userdata.img as the command "fastboot update image-xxx.zip" does not touch the userdata on your device.
*Disclaimer: I have followed the above to the T and moved from Stock 4.1.2 (yakju) to Stock 4.2 (takju) with Unlocked Bootloader, but you follow these at your own risk.
Hit Thanks, if this has helped you.