Google announced that Android 4.2 Jelly Bean update already available as over-the-air update update for Nexus 7. It is will become valuable update for the Nexus 7 since Android 4.2′s features will give different taste technology. But, since it’s new announcement so there are no valid data about how many Nexus 7 user already update their OS system.
Typically it will take times to distribute the new Android 4.2 Jelly bean to all Nexus 7 users around the world. As it happened when Google released Android 4.1.1 update in the past period. It took about three months to distribute the update all over the world. So some Nexus 7 owner should be patience and wait for OTA update.
But if you like some tricks, you can try this step. But please be noticed, it’s not always works, but worth to try with your own risk :). The overall steps will be like this. Go to Setting ->Apps->All. Then run Google Service Framework. Select Force Close button. The next step is go to Settings menu. Choose About->System update.
Perhaps that trick may works for you since it also will depend on the update availability. Sometimes the availability is different among the countries.