Google is updating Nexus devices to Android 4.2 Jelly Bean via an over-the-air update today.
Those with the GSM version of the Galaxy Nexus, who don’t feel like waiting for a rollout, can grab the firmware manually from Google’s servers now, while the v4.2 ROM for the Nexus 7 is also available.
Galaxy Nexus owners need to download the “takju” ROM (.zip), and then install manually or use a ROM Manager. Nexus 7 owners have a similar process: download the Jelly Bean update (.zip), and then manually install or use a ROM Manager.
As SlashGear noted, ClockWorkMod’s ROM Manger in the Google Play Store is a free and easy solution. However, if the above steps confused you, just wait it out. The update will land over the next few days.