A few days ago we let you know of the Android 4.0.3 Ice Cream Sandwich update that Google was preparing for the GSM version of the Nexus S.

As with most OTA updates, this one is being distributed in a staged release, and it’s fairly large (128MB or so). If you’d rather not wait, or don’t want to download the whole update on your Nexus S, you can flash the update by following these simple steps.

It should be noted that this is the official ICS update and requires that you have the stock recovery image to properly apply. If you’ve got a custom recovery or custom ROM loaded you’ll probably need to do a full wipe, then restore the stock recovery image before going any further.

0. Assume the risk that flashing a ROM bring along with it

1. Download the OTA .zip file here (this is the T-Mobile version)

2. Rename it to update.zip

3. Copy the file to your Nexus S’ internal storage

4. Turn your Nexus S off

5. Hold down the volume-up and power buttons to reboot to the bootloader

6. Use the volume key to select recovery, then press the power button to confirm

7. When you see a warning triangle and arrow, hold the power button and press volume-up

8. From the new menu select “apply update from /sdcard”, and choose update.zip from the list.

9. The update will apply, when finished choose “reboot system now”

Enjoy your Ice Cream Sandwich!

Source: XDA