This tutorial will show you how to manually Install Android 2.3.4 On Google Nexus S. Just follow the instructions below
Step 1: First, Download Android 2.3.4 Gingerbread (GRJ22-from-GRI40)
Step 2: Connect your Google Nexus S to your computer via USB cable
Step 3: Copy the file you downloaded from the above link and paste it in the root of your Internal SD Card. [Note: Don’t extract or rename that file]
Step 4: Disconnect USB cable connection and turn off your Nexus S.
Step 5: Boot your Nexus S into recovery mode by pressing and holding the “Volume Up” key and “Power” button simultaneously.
Step 6: Once in the recovery mode, choose apply update from /sdcard
Step 7: Select the file you downloaded (GRJ22-from-GRI4…zip) and “Power” button to flash it.
Now wait till you device is being updated. After it’s done, your Nexus S will reboot and you will be on Android 2.3.4 Gingerbread.