Not sure what qubes defaults nowadays are, but if you see grub (menu with various qubes-related booting options) when starting, you can simply select the “UEFI boot settings” and you’ll be rebooted into UEFI BIOS.
To add in the high unlikelihood that if you’re using Coreboot+Heads, press any key on your keyboard when it tells you “Automatic boot in 5 seconds unless interrupted by keypress...” and go to Options ->.