1. Download the latest factory image for the phone from https://developers.google.com/android/nexus/images
  2. Extract the files, including the inner zip file
  3. Copy the radio, recovery, boot and system.img files to the directory containing fastboot (C:\Program Files (x86)\Android\android-sdk\platform-tools)
  4. Reboot the phone in fastboot mode (hold volume down and power button)
  5. 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
  6. Type: fastboot flash radio <radio_filename.img>
  7. Type: fastboot reboot-bootloader
  8. Type: fastboot flash recovery recovery.img
  9. Type: fastboot flash boot boot.img
  10. Type: fastboot flash system system.img

If all went well, you should now be able to reboot and be running the new version!