If you need to enter the BIOS: bring up the terminal, and write
`systemctl reboot --firmware-setup`
This reboots systemd-powered systems to the UEFI menu. May not work on other init systems, but SteamOS is based on Arch with systemd.
Shouldn't matter what you have as a bootloader, by my understanding it should be communicating with the UEFI directly to pull it off and going over any bootloader's head.