Google has just started rolling out the official Android 4.1.1 Jelly Bean update for the Nexus S via OTA in certain regions of the world. The OTA update will however take some considerable amount of time to be available in different regions of the world, and to the different variants of the handset.
If you cannot wait for the OTA update to be available in your region, and want to update your Nexus S to Jelly Bean manually, follow the instructions below.
The steps below are only valid for the I9023, I9020T and the I9020A variant of the Nexus S. Also, make sure you have root access with a custom recovery like CWM installed.
Step 2: Once the download is completed, transfer the zip file(s) to your Nexus S. Reboot your phone into CWM Recovery mode by using ‘Quick Reboot’. Navigate to the ‘wipe data/factory reset’ option first using the volume keys, and select it by pressing the Power button. Then navigate to ‘Mounts & Storage’ and from there select the ‘format /system’ option.
Step 3: Finally, select the ‘install zip from SD card’ option and then select the Jelly Bean OTA zip file that you downloaded in Step 1. If you also want root access, flash the superuser zip file as well.
Credits to kibmcz!