With the Nexus S on, plug it onto your PC and mount the phone storage.
Copy the file to root of the phone storage.
Power down the phone and disconnect USB cable from phone.
Next you need to boot the phone into Recovery Mode, by holding down the “Volume Up” and “Power” button at same time.
Now in Recovery Mode navigate down to “Recovery” using the volume button for navigation and power button to select.
A “!” graphic will be showing, hold the “Power” button and then press the “Volume Up” button to get recovery options. (Note: you might have to press power and volume button a few time for it to respond)
Navigate down to “Apply update from /sdcard” and press power button.
Select the update file you copied to the root of phone storage then press power button to flash it.
WAIT for the upgrade to complete, then reboot phone for the upgrade to take effect.
Congratulations you now have Gingerbread firmware installed.