If you have rooted your Nexus 7 and flashed a custom ROM and want to restore the tablet to its original factory flavor, here is the tutorial about getting back the stock version of Android. Notes that all data on your device will be fully wiped.
How to restore Nexus 7 factory settings
* Make sure you have setup ADB and fastboot on your computer.
* Download the 4.1 (JRN84D) image files from Google to your computer.
* Extract the downloaded file and copy the following files into your "fastboot" folder whcih contains the fastboot files:bootloader-grouper-3.34.img, flash-all.sh, flash-base.sh, image-nakasi-jrn84d.zip.
* On your Mac, Linux or Windows system, open the terminal and navigate to the above folder by using cd command.
* Powering down N7 and holding Volume Down + Power until you see the bootloader screen, then connect to computer via USB cable.
* Enter "fastboot devices" command (Mac uses "./fastboot-mac devices"). If you see list of attached devices, it means your device is recognized and fastboot drivers are installed.
* Enter one by one to wipe all old data on the device.
fastboot erase boot
fastboot erase cache
fastboot erase recovery
fastboot erase system
fastboot erase userdata
* Flash the stock bootloader by enter:
fastboot flash bootloader bootloader-grouper-3.34.img
* Make a reboot.
* Flash the factory image into the tablet, including the stock system, kernel, and userdata partitions.
fastboot -w update image-nakasi-jrn84d.zip