While googling, I found this link that guided me step-by-step to get Android Jelly Bean 4.1.2 into my Nexus 7.
But there is a quick little trick that will push the OTA update into my Nexus 7 immediately.
1. Go to Settings -> Apps -> All
2. Find Google Services Framework.
3. Tap on Force Stop.
4. Select Clear Data.
5. Head back to the Nexus 7’s Settings, go to About, and check for a System Update.
This trick was taken from this site.