How to manually upgrade Nexus S from Android 2.3.6 to Android 4.0 Ice Cream Sandwich

If you own T-Mobile’s variant of Samsung Nexus S (i9020t) and don’t want to wait for the official OTA Android 4.0.3 update, you can install it manually. The procedure below will upgrade stock Android 2.3.6 build GRK39F to Android 4.0.3 build IML74K.
With a rooted phone the easiest update process is to use ROM Manager (Download ROM -> Stock Images -> Nexus S 4.0.3).
If you don’t have root, follow these manual steps (if you are new to the update process, check out this slightly outdated, but detailed update guide).

• Download the update package.
• Rename downloaded file to update.zip.
• Copy the update.zip file to the phone’s internal storage.
• Turn the phone off, press and hold Volume Up and Power to boot into the bootloader.
• Using Volume Up/Down select “recovery” and press Power button.
• After the exclamation point in a triangle appears, hold the Power button and press Volume Up.
• Using Volume Up/Down select “apply update from /sdcard” and choose the update.zip.
• After the update process is finished, select “reboot system now” and press Power.