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If you are on Developer Mode (not developer channel) you can force an update to Chrome OS by typing the following command in Chrome OS shell
update_engine_client -update
Get into dev mode hold esc refresh & power button then press Ctrl & D together
Device should start in dev mode now
Developer mode requires you to turn off OS verification in most cases, making it an unsafe option
You should only turn it on if you're sure of what you're doing