The normal way to enter UEFI BIOS on a Windows 10 machine is to use Shift-Restart and then select it from the Windows 10 Recovery Environment. But what if Windows 10 won’t start? There are other ways to get to BIOS, for example using “Shutdown /r /o” from the command prompt, but all of them require the operating system to boot in order to use them. In the old days of course you could just enter BIOS at POST time by pressing F2 (on Dell and many other computers) or ESC (on HP) or Insert (on Asus), but apparently that was too easy for the people who designed the UEFI specification.
We had this very problem recently on a machine on which we had to change the boot parameters because we wanted to boot it from an external, non-UEFI disc. We turned off Secure Boot and changed the boot style to Legacy Boot instead of UEFI Boot. This helped us boot from the external CD that we were using in the repair. But then when we restarted the machine afterward, it couldn’t find Windows 8.1 (the same thing would happen in Windows 10) or even the hard drive for some reason.
A reasonable motherboard will throw you into BIOS if it has problems finding any boot information, at least that is what ASUS does. Alas we were stuck with “No Boot Device”. Booting from a Windows 10 disc and opening the recovery environment was our hopeful plan but that didn’t work because the choice to open BIOS is not provided there like it is in the installed version.
Eventually we found the key in the Sony “Assist” button. If you hold it down and start the machine with the power button, it drops you into Sony’s built-in diagnostic environment, from which one of the choices is to enter BIOS.