To enable developer mode you need to press ESC + Refresh + Power at the same time.

Reboot your device and press CTRL + D to enable developer mode.

Disable OS verification by pressing CTRL + D again.

Press right alt + t boot chromeOS in developer mode.

Open a terminal (ctrl + a) and type shell to enter the shell.

Type sudo crossystem dev_boot_usb=1 dev_boot_legacy=1 to enable legacy boot.

Type curl -L -O https://mrchromebox.tech/firmware-util.sh && sudo bash firmware-util.sh to download and run the script.

Select option 3 to install the full ROM firmware.

Reboot your device and press CTRL + L to boot from USB.

Install your favorite linux distro.