We need to enable Developer Mode to install another BIOS and operating system. To enable Developer Mode, we must first invoke Recovery Mode.

To start Recovery Mode, power up the Pixel. At the login screen, hold down the Esc (top-left-most key) and Refresh keys (three keys to the right of Esc on this model), then press the Power button (top-right-most key) for a moment, until the keyboard's backlight turns on, and you see the display blank and restart.

Once you see the exclamation mark and the message "Chrome OS is missing or damaged," press Control+D

You'll see one last check that you really want this (you do!): "To turn OS verification off, press Enter." Press Enter. As it says, your laptop will reboot, and your data will be wiped.