Step 1: Unlock bootloader, root and flash ClockworkMod Recovery on your Nexus S.
Step 2: Download Android 4.1.1 Jelly Bean (JRO03E) [Direct Download – Google]. This .zip file is for update to Jelly Bean from Android 4.0.4 (Build: IMM76D) on Nexus S models i9020T / i9023. Some users have reported that this .zip file also works on i9020A, even though it’s not supposed to.
Place this directly under /sdcard on your Nexus S’s internal storage.
WARNING: DO NOT use this .zip file if you are not on Android 4.0.4 with the specific build number previously mentioned. or maybe try to install android 4.0.4 whit this TUTORIAL
Step 3: Reboot into ClockworkMod (CWM) Recovery by shutting down your Nexus S, and then pressing and holding Volume Up + Power buttons.
Step 4: Take a NANDroid backup. This will come handy later on if you find any issues with the Jelly Bean ROM.
Step 5: Navigate to install zip from sdcard > choose zip from sdcard > [.zip file you downloaded in Step 2]. This will flash Android 4.1.1 Jelly Bean to your Nexus S.
Step 6: From CWM’s main menu, select reboot system now.
If you followed the steps correctly, your Nexus S will now boot into an oven fresh copy of Android 4.1.1 Jelly Bean.
Now, if you have any question about this tutorial, please head yourself to XDA Developers. Or comment below!