AndroidMixer

Grab the OTA package
Rename it to update.zip. This isn't needed for the Nexus S bootloader, but it makes things easy, if that's how you prefer them.
Copy it over to the internal storage on your Nexus S.
Power off, then hold volume up and power to reboot to the bootloader.
Using the volume key to navigate, select recovery, then use the power button to confirm
When you see the warning triangle and arrow, hold the power button and tap volume up. You'll see a menu.
From the menu, select "apply update from /sdcard", and choose update.zip from the list.
Let it do its thing and update your system, radio, and other partitions. When finished, choose "reboot system now"
Enjoy!

http://www.androidcentral.com/how-manua ... m-sandwich