Upgrade your Nexus 7 (Wi-Fi) to 4.2 Manually
Google has released 4.2 for Nexus 7 on WEB. Maybe someone has not received OTA notification for upgrade. If your device got some problem on your flash OS because of certain reasons, you can also recover your Nexus devices by the image that Google released on WEB.
Note that you can select the image rom for your Nexus device. You need to download the rom code with the same model for your Nexus device and then you can upgrade your Nexus device. For different Nexus model, you can take a look on web site - https://developers.google.com/android/nexus/
You can follow the instruction below to upgrade or restore your Nexus. Note, you need to backup your personal data in advance, because using this instruction would erase all your data.
1. Reboot the device as fastboot mode by command - adb reboot-bootloader.
2. Unlock your bootloader by command - fastboot oem unlock
- The warning screen would bring up, please read it and make decision if you agree or not. If you don't agree, you cannot do the following action.
3. Using volume up or volume down to navigate the options and press power key to execute the selected option. If you agree the warning shows on screen, move the option to agree or ok or accept and then press power key to confirm the option.
4. Download the image from Google - https://dl.google.com/dl/android/aosp/nakasi-jzo54k-factory-973f190e.tgz for 4.1 and https://dl.google.com/dl/android/aosp/nakasi-jop40c-factory-6aabb391.tgz for 4.2.
5. Unzip all the file to the folder and double click flash-all.bat within the folder.
6. Don't touch your Nexus 7 until flash is successfully.
7. After the flash process is done, lock your bootloader to secure your Nexus by command adb reboot-bootloader and then perform fastboot oem lock.
Above instruction may be used to recover your other Nexus device. You can look at web sit https://developers.google.com/android/nexus/images?hl=zh-TW for more information.
In case you don't receive the OTA upgrade notification, another way to upgrade your Nexus 7 is as follows,
Google Service Framework stop and Clear Cache. And then press the upgrade check on your Nexus. This is a method I got from someone's talk. You may be able to try it in case you just want to upgrade your Nexus 7. Why the method can be workable? I guess it's a bug for Nexus 7 OTA upgrade.
Bootloader unlock, https://groups.google.com/forum/?fromgroups=#!msg/android-building/Ld5FoFinN7k/xcZPFBikw0MJ
Nexus Factory Image, https://developers.google.com/android/nexus/images?hl=zh-TW