Google pushed out 4.0.4 update for its Google Nexus S, Galaxy Nexus and Xoom users.
Here is info for updating your Nexus S without voiding your warranty.
Before you go into this check for update if available by dialling *#*#CHECKIN#*#* (*#*#2432546#*#*)
Also you can go in Settings -> About Phone -> System Updates
If it ain't there .. no worries it is here :D above steps were just to save you from all these steps :)
Download the update from here
Step 8: Now select “reboot system now” option. That’s all you need to do.