You must have an unlocked Nexus 7 to flash this firmware, else use the toolkit linked below to unlock it.
Disclaimer: The author and AndroidPure are not responsible for any damages you may incur.
Warning: You will lose all your data, so take a backup before proceeding. You will also lose Root access.
How to manually install the official Android 4.3 firmware on the Nexus 7
1. Download the official Android 4.3 (JWR66V) Image for the Nexus 7 from here.
Note: Nakasi is for Wi-fi only tablets and the Nakasig is for 3G version.
3. Run the Toolkit and make sure you install the drivers for the device by clicking on the “Full Driver Installation Guide – Automatic + Manual” button. If you already have the drivers installed, just skip this step and go to step 4.
(I have used a different Toolkit before and I already had the drivers. So I skipped this)
4. Connect the USB Cable to your PC and plug in your Nexus 7.
5. Now in the toolkit, under the “Flash Stock + Unroot” option there are two options. Check the second one. Device is on/normal.
6. Click on the “Flash Stock + Unroot” button and click ok.
7. Select “Other/Browse” in the first set of options and “I downloaded a factory image…..” in the second one. Click ok.
8. Browse to the folder where you have downloaded the firmware and select it.
That is all.Wait for the Toolkit to finish flashing the firmware and wait for the following window.
Here is a screenshot of Android 4.3 (JWR66V) running on my Nexus 7.
In case the device fails to boot. Retry from step5 and choose “Softbricked/Bootloop” and follow the other steps.
via: XDA matt54