How to Use “adb sideload” to Update a Nexus Without Root or Custom Recovery
1. Install Android Debug Bridge and make sure your device is recognized in when typing "adb devices".
2. Download the OTA zip file.
3. Type "adb reboot-recovery"
4.Your device will boot to an Android logo with a exclamation mark. To jump past this, you either have to press power and then volume up or vice versa. Try both ways.
5. With Volume Down, highlight “apply update from ADB.”
6. Open up Cmd Prompt and type "adb sideload .zip" (drag the zip to the cmd prompt)