Manually Update Galaxy Nexus To Android 4.1.2

You will need: A Galaxy Nexus flashed with the takju firmware. If you brought your device from Google Play, you’re running takju. Ensure your device is rooted / bootloader unlocked, and ClockworkMod Recovery flashed.

Step 1: Navigate through Settings > About > System Update to ensure you’re not wasting your time.
Step 2: Download the required file from Google.
Step 3: Transfer the .zip file you downloaded above to /sdcard (root directory).
Step 4: Reboot into ClockworkMod Recovery.
Step 5: Take a nandroid backup.
Step 6: Select install zip from sdcard > choose zip from sdcard > [.zip from Step 2]