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.
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.
