1. Download Ice Cream Sandwich Android 4.0.3 update.
2. Connect Nexus S to a computer using USB and copy the update.zip to the root of SD card. (Note: do not put in a folder because it will mot work.)
3. Turn off the Nexus S and turn it on again while you press and hold down the 'volume up'.
4. Use the volume keys to steer the 'RECOVERY' mode and press the 'power'.
5. If you are seeing the emergence of an exclamation point on the screen, press the 'power' and follow the 'volume up'.
6. Select an option to 'Update from SD card', select the zip file you have dowload in stage # 1. Press he 'power' to confirm.
7. Later updates will run on Android 4.0.3 Nexus S. Once completed the Nexus S will bootup to the latest Android 4.0.3 Ice Cream Sandwich.
To check the presence of OTA updates please go to the Menu button and select Settings> About phone> System updates. Make sure you get a strong WiFi signal ICs to download updates on your Nexus S.