- Download the latest factory image for the phone from https://developers.google.com/android/nexus/images
- Extract the files, including the inner zip file
- Copy the radio, recovery, boot and system.img files to the directory containing fastboot (C:\Program Files (x86)\Android\android-sdk\platform-tools)
- Reboot the phone in fastboot mode (hold volume down and power button)
- Check that device is found while connected to PC by typing: fastboot devices, you should see a response with a uid and the word fastboot if it is in fastboot mode
- Type: fastboot flash radio <radio_filename.img>
- Type: fastboot reboot-bootloader
- Type: fastboot flash recovery recovery.img
- Type: fastboot flash boot boot.img
- Type: fastboot flash system system.img
If all went well, you should now be able to reboot and be running the new version!