Google Nexus S — How to downgrade from ICS ???

Well if the phone already has ICS installed, I'm pretty sure it also has Clockworkmod Recovery(aka. CWM or CWR). Try this:
Turn off your phone.
Turn it on by HOLDING DOWN the power button AND the volume up button. After 2-3 sec. you should see a menu. Scroll down 2 clicks with the volume down button, you should land on 'Recovery '. Press the power button to enter CWR. (Clockworkmod Recovery)
If this procedure takes you to another menu, then you have a functioning CWR. And if so, I would recommend trying one or more of the fine roms in this forum under 'Android Development'. Many of these roms have a lot of features and fixes that you would miss once you've had them.
Try it out, and happy flashing.
PS. To flash most roms, download the rom on your mobile. (it's a zip-file) and go to CWR. Choose 'wipe data/factory reset' and then go to 'advanced' and there you should choose 'wipe dalvik cache'. Go back, choose 'install zip from sdcard' and then 'choose zip from sdcard'. Locate the rom you downloaded and choose it. This will flash the rom. Go back and choose 'reboot system now'. The first boot on any rom will allways take longer than usual. Usually about 2min. Enjoy!