You must have 4.1.2 stock firmware (This is not rom)
Download the update zip from google server http://android.clients.google.com/pa...K.094f6629.zip
Mirror 1 http://download.tbridge.kr/094f66293...K.094f6629.zip
Mirror 2 http://www.mediafire.com/?cmz138n95r95cez
Mirror 3 http://d-h.st/GEr
If you have a custom recovery, you can just copy it to the device, reboot to recovery, and flash directly as you would any other zip. You're done.
If you have the stock recovery, carry on.