It was just last week that Google first started rolling out the Android 4.1.2 update for their Google Nexus 7 and now its making its way to the rest of the Nexus devices including Google Galaxy Nexus and the Google Nexus S. Once the update reaches your device you should receive a notification of its availability. Keep in mind that it may take some time before the update reaches your device as the roll out finds its way to all the devices.You can also manually check for it by heading over to Settings > About > System Update or you could try to force the update by following this guide.
The update will mainly offer bug fixes and performance improvements for the Nexus S, along with the ability to expand/collapse notifications with one-finger gestures.