How to manually "OTA" update your Galaxy Nexus from 4.1.1 to Android 4.1.2

The Android 4.1.2 Jelly Bean update is gradually rolling out OTA (over the air), but if you are impatient you can update manually. I had previously unlocked my boot loader and installed su (superuser), but I otherwise like to keep things as stock possible (no custom ROMs, no kernel hacks, no custom recovery manager).  If you have a similar setup then this upgrade is very simple; it can actually be completed without connecting the phone to a computer, so it really is OTA.

Preconditions:

1. You have a GSM Galaxy Nexus (i9250)
2. You are currently running the latest Android 4.1.1 (JRO03C)
3. You have the stock yakju or takju ROM.  (You can use this app to check)
4. You have root access (su/super user is installed) 
5. You have a terminal application installed (e.g. Android Terminal Emulator)
6. You *have not* replaced the stock recovery system/partition with a custom recovery (like Clockwork Mod)
7. You have 15 - 20 minutes to spare

P.S. Over time, the OTA files for other stock builds will become available (but only for Google supported Nexus devices). You can monitor this forum post at xda-developers if you have a non-yakju ROM. I am personally waiting for the takju file myself so I will update this post when it made available.

Update: The takju file is now available, the instructions are the same, just substitute this link takju-JZO54K-from-JRO03C for the OTA file and this file name: 06fa1976791d.signed-takju-JZO54K-from-JRO03C.06fa1976.zip where relevant.

Steps:

1. Download the yakju 4.1.2 (JZO54K) from 4.1.1 (JRO03C) OTA update file directly to your phone (you are downloading directly from Google, not from this blog).  The file is about 15MB so you may want to do the download over wifi.  
2. Copy the update file to the /cache folder (this requires super user permission)
• Open a terminal window
• su
• cp /sdcard/Download/24a9a760f32e.signed-yakju-JZO54K-from-JRO03C.24a9a760.zip /cache/
3. Boot into recovery mode
• Power off your phone
• While the phone is off, press and hold Volume Up + Volume Down + Power
• Use the volume button to scroll through the menu options and press power to select "Recovery Mode"
• Once the Recovery Mode screen has come up (green android with red warning sign), press  Power + Volume Up to bring up the menu
4. Install the update from the /cache folder
• Use the Volume button to select the "apply update from /cache" menu option and press power to invoke it
• Use the Volume button to select 24a9a760f32e.signed-yakju-JZO54K-from-JRO03C.24a9a760.zip and press the power button to start the update
• Once the install is complete, select "reboot system now"

Warning: The last upgrade caused me to lose root access. Apparently the setuid bit was removed from /system/bin/su.  To prevent this you can try a proactive approach by using an app like OTA RootKeeper or see my previous for details on how to restore root access.

Discalimer:

If you have anything important that isn't stored in the cloud, please back it up before you start. You never know what might go wrong.  This "worked for me", but your mileage may vary. I take no responsibility if you phone stops working, explodes or tries to take over the world. Caveat Emptor!