If you are unable to boot to the BIOS in either Windows 8 or 10, then it’s probably because you’ve got Fast Start-up turned on. To fix this and boot to the BIOS, boot to Windows, then go to the Control Panel -> Power Options, then click Choose what the power buttons do on the left hand menu. Untick Turn on fast start-up (recommended) – note that you may have to click Change settings that are currently unavailable if you can’t turn off fast start-up.
After saving the setting, you should be able to reboot and get then boot to the BIOS.
Alternatively if you don’t want to turn off fast start-up, you should be able to get to the BIOS by power in Windows, then restarting the computer (instead of starting from shutdown).
If those options fail and you still can’t boot to the BIOS, consult your motherboard’s manual to find out how to turn off fast boot or see if there could be something else preventing you from booting to the BIOS. In my case, Windows thought that fast boot was turned off, but my Intel motherboard thought that it wasn’t – guess who won!