Follow these instructions:
> Go to your home screen; make sure Chromebook isn't open.
> Hold down on esc + F3 (esc), then press power.
> Wait for title screen to say "Chrome OS is missing or damaged. Please insert USB stick."
> Press Ctrl+D, and if prompted, press Enter.
> The device will reboot, then you will follow the on screen instructions.
I hope this helps, have a great day! :)