Samsung Nexus S i9020a: Update to ICS 4.0.4 from 2.3.4/2.3.6 (without rooting)

Update from 2.3.4 (GTJ61)/2.3.6 (GRK39C) to 2.3.6 (GRK39F)

1. Download this file
2. Rename the downloaded file to update.zip
3. Put it update.zip you have downloaded on your sd card. 
4. Turn off your device 
5. Go to Bootloader (press&hold volume up button then press&hold power button) 
6. When in Fastboot mode navigate to Recovery using volume up/down buttons 
7. Press the power key to select Recovery 
8. If you see a warning logo, press the power button and then press volume up button then release both to go to the Main Recovery menu
9. From the menu, select Apply update from /sdcard, and choose update.zip from the list. 
10. When update is finished go back and select reboot system now. 

Update from 2.3.6 (GRK39F) to 4.0.4 (IMM76D)

1. Download this file
2. Use the above steps again to update.

*** The same procedure should work for other devices as well but the file would be different. You will have to find the right update file.