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the update process, check out this slightly outdated, but detailed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.