Can't boot from USB on chromebox

1st Official Post

    I want to remove libreelec from my Lenovo chromebox and reinstall chrome os (giving the box to a friend). My drive was wiped and no longer has chrome os. I also dont have a back up of chrome os. I put ubuntu ISO on a USB stick. How do I boot from it? The write protect screw is not installed if that matters. I tried holding in the reset button on the back of the box as It booted, but it didn't work. It keeps booting into librelec. I tried holding it it for just a second, holding it for about 30 seconds and pressing it repeatedly. Any ideas? Thanks!!