Updated!!!
This should be the last update to the guide(Guide) How to flash from yakjuxw ICS/JB to yakju JB V3.1With the release of Galaxy Nexus Android Jelly Bean Factory Images, you are now
no longer need to flash to yakju ICS from yakjuxw and then OTA to get Jelly Bean...You can now
directly flash to yakju Jelly Bean from any yakjuxw ICS
* Just adjust yourself on the version you are flashing to *Note: 1. About the build number
Build IMM76K (Android ICS 4.0.4 yakjuxw). Please bare in mind that our Gnex is
GSM set - Maguro, even they have the same build number with CDMA one,
please ignore it!!
2. No matter which
yakjuxw version you are using (4.0.1 - 4.0.4, 4.1- 4.1.1), this guide
should work.
3.
(Important!) For
Step 5 and 9, please select according to
the version you are using (ICS / JB). If you are using ICS right now, please select
Android 4.0.4 (Build IMM76I). If you are using JB right now, please select
Android 4.1.1 (Build JRO03C)4.
(Important!) For
Step 13, please select according to
the version you are flashing to (Usually the latest one). Let say it's 4.2, then select 4.2 and vice versa.
Method 1 (Using Gnex Toolkit): -
Highly recommended 1) Download v10 Gnex toolkit or the latest one
Here (V10 is the latest at the time of writing)
2) Install it
(Gnex toolkit)3) Go
Here and download Factory Images
"yakju" for Galaxy Nexus "maguro" (GSM/HSPA+) version 4.2 OR the latest one (4.2 latest available on the page at time of writing)
(DO NOT EXTRACT IT!!)4) Navigate to ''C:\Galaxy Nexus toolkit\ put_google_factory_image_here\'' in your HDD and place the downloaded file.
5)
(*Check Note 3*) Now open the Gnex toolkit, select
Android 4.0.4 (Build IMM76I) for ICS OR Android 4.1.1 (Build JRO03C) for Jelly Bean from
GSM Model (Yakju-Maguro)6) Then, select
"Install/uninstall galaxy nexus drivers on your PC" then your
operating system then
"Install drivers to your PC" (without plugging in the phone 1st) and
follow on screen instruction.
<--- Very important to make sure driver is installed properly or it wont work6.1) If u can't get the driver install using Gnex toolkit, please check note 2 below.7) Reboot computer.
8) Open toolkit again, turn off phone. But this time, turn on phone again but
hold down all buttons at the same time. (the vol + and vol - and power button)
9)
(*Check Note 3*) Now, plug ur phone into computer. In toolkit, now select
Android 4.0.4 (Build IMM76I) for ICS OR Android 4.1.1 (Build JRO03C) for Jelly Bean from
GSM Model (Yakju-Maguro) agn.
10) Select "
Unlock or Re-Lock your bootloader" then "
Unlock bootloader" and follow on screen instruction.
11)
(Optional, If your phone is still in fastboot mode.) After done that, turn off phone once agn. But this time, turn on phone again but
hold down all buttons at the same time. (the vol + and vol - and power button)
12) Select
"toolkit settings" (should be around the bottom), go to
"model/ Build selection page".
13)
(*Check Note 4*) This time please select
Android 4.2 (Build JRO03C) OR
the version you are flashing to from
GSM Model (Yakju-Maguro).
14) Select
"Download, extract and flash google factory stock rom" then
"Flash Google factory image" and then follow on screen instructions
15) If everything works well.....Your phone boot up and it
might take a while for the 1st boot. And you will be on
yakju 4.2 Jelly Bean OR the version you are flashing to.
Method 2 (Using manually adb mode): -
Only for advanced user » Click to show Spoiler - click again to hide... «
http://forum.xda-developers.com/showthread.php?t=1626895
Remark 1:
Im not responsible for anything happen to ur phone. Use it at ur own risk.Remark 2:
Manually install using universal naked driver 0.7
Guide: Here
Driver File: HereRemark 3:
If u still need the old guides, check the following links:[Guide] How to flash from yakjuxw ICS to Stock Yakju ICS v2.1[Guide] How to update ur Gnex stock Yakju ICS to Jelly BeanI have not used GNex Toolkit for very long time already, so report back if you find any problem 
updating 4.1.2 to 4.2 but stuck when reboot. screen shows the android with its stomach opened and a ! sign . how @.@
up down vol power (fastboot) and select recovery. if u see cwm then u nid to get rid of it.