Anyway, if you are patient enough to wait till Google launch the over the air update of the Android ICS, then do so. Or you can use this official Ice Cream Sandwich Rom to upgrade your GSM Nexus 5 to Android 4.0.3.
1. Download the ICS Rom from this link.
2. Rename ICS to whatever name that is easily remember, but let's keep it as simple as possible, rename it update.zip.
3. Copy the renamed file to the root folder -- internal storage of your Nexus S.
4. Turn your phone off.
5. Hold both "Volume Up" and "Power" together in order to reboot to the bootloader.
6. Use your volume Up and Down, and select recovery, and use the power button to confirm.
7. When you see the warning triangle and arrow, hold the power button and tap volume up. You'll see a menu.
8. Choose "apply update from /sdcard" from the menu, and select update.zip on the list.
9. Let it do its job and updating the system of your, radio and partitions. Once all the jobs are done, select "reboot system now"
Well done now your GSM Nexus S is running on Android 4.0.3 (ICS), congratulation!
Via: Android Central.