Google has started rolling out the Jelly Bean update for Nexus S smartphone. The update is now rolling out over-the-air and is available for GT-I9020T, GT-I9023 variants.
This JRO03E update will take Nexus S to Android 4.1.1 and is 114.4MB in size.
You can check out the full Jelly Bean change-log here.
You will also use custom recovery to install the update manually, which is available for download from Google.