How to fix android device won't update to 4.2 issues
* Performing a factory data reset on the device. That will delete all your data. Touch Settings > Backup & reset > Factory data reset, and select "Reset tablet" and then "Erase everything". Power down the N7, Press and hold Power + Volume Down into Recovery mode, and select "wipe data/factory reset".
* If still not working, try to flash the factory images for Nexus 7.