In theory I can go to Settings, Update and Recovery, Recovery, Troubleshoot, Advanced Options and there I should see the UEFI Firmware Settings but it is not there
1)
IMHO, you have to reformat your system hardrive and re-install the win8.1 x64 in UEFI mode (you can see the UEFI option in bootmenu,if your win8x64 DVD or usb thumbdrive is support UEFI)
please notice that, your hardisk will be re-formatted to GPT, instead of MBR.
that's all.
You can see the UEFI firmware setting option the windows Settting > Update > blah...blah..blah.. now.
2) or, start up the window normally, and then using "restart" option to restart the window , instead of "shutdown".
and then, you can enter the BIOS normally by click "DEL" button (or other button, pls check your motherboard manual for the detail).