How to install an Android factory image onto a Nexus device

Flashing an Android system image can be complicated. It will also reset your device back to factory settings. Good thing there are easy step-by-step instructions for doing it.

First, head to Google’s Factory Image site and find the latest version for your device (currently Android 5.0.1/5.0.2 depending on the smartphone/tablet).

To flash a system image:

1. Download the appropriate system image for your device below, then unzip it to a safe directory.
2. Connect your device to your computer over USB.
3. Start the device in fastboot mode with one of the following methods:
   • Using the adb tool: With the device powered on, execute:

     adb reboot bootloader

   • Using a key combo: Turn the device off, then turn it on and immediately hold down the relevant key combination for your device. For example, to put a Nexus 5 (“hammerhead”) into fastboot mode, press and hold Volume Up + Volume Down + Power as the device begins booting up.
4. If necessary, unlock the device’s bootloader by running:

   fastboot oem unlock

   The target device will show you a confirmation screen. (This erases all data on the target device.)

5. Open a terminal and navigate to the unzipped system image directory.
6. Execute the flash-all script. This script installs the necessary bootloader, baseband firmware(s), and operating system.

Once the script finishes, your device reboots. You should now lock the bootloader for security:

1. Start the device in fastboot mode again, as described above.
2. Execute:

   fastboot oem lock

Some recent Nexus devices may throw an error when attempting to flash-all. In that case, you’re going to want to extract the .zip file containing all the individual .img files and flash them individually.

Here is the order to flash them:

• fastboot flash system system.img
• fastboot flash recovery recovery.img
• fastboot flash cache cache.img
• fastboot flash boot boot.img
• fastboot flash userdata vendor.img (if it’s included)
• reboot

Locking the bootloader does not wipe any data, but when you want to flash it again, you must run fastboot oem unlock again, which will wipe the data.

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Remember, flashing a factory image overwrites any data on the device, including media, photos and documents.

[source]Google[/source]