If you're a Sprint customer using either the Galaxy Nexus or the Nexus S 4G, you're in luck. Sprint has started rolling out updates to Android 4.1.1 Jelly Bean. The update is delivered to users' phones via an over-the-air (OTA) update. Owners of the Samsung-made Galaxy Nexus can use the steps found on the update page to download and install the update.
Nexus S 4G users are also getting upgraded to the goodness that is Jelly Bean. This update is also available over-the-air, but could take a while to reach all users as a company employee posted on the Sprint forums saying that it would be a "slow roll out." Keep checking back often to see if you can get the OTA update.