How to Disable UEFI Secure Boot in Windows 8/ 8.1
UEFI (Unified Extensible Firmware Interface) is a standard firmware interface for new PCs pre-installed with Windows 8/10, which is designed to replace BIOS (basic input/output system).
Basic steps: Access UEFI BIOS settings and disable "Secure Boot" option, then change "Boot List option" as "Legacy", and enable "Load Legacy Option Rom", then follow a traditional method to boot computer from USB device or CD-ROM.
There are several ways to access Windows 8's boot options menu. The easiest one to find is in the PC Settings application – press WinKey+C to reveal the Charms bar, click Settings, and select Change PC settings to access it.
Part 1: Access UEFI BIOS by PC Settings
Move your mouse to the upper-right side of PC window (or press Win+C) to open the Settings Charms bar.
Then click Change PC Settings in the bottom right.
1) In Windows 8, from PC settings screen, you will find the General option listed within the left column and clicking it you can see Advanced startup option.
Click Restart under Advanced startup option.
2) In Windows 8.1, from PC Settings screen, you will find the Update and Recovery option listed within the left column and clicking it produces three additional sub-categories — Windows Update, File History and Recovery. Clicking the Recovery sub-category, you will see Advanced startup option.
Click Restart Now under Advanced startup option. Your computer will restart to the Windows 8s boot options menu.
Now you will be presented with the Choose an option screen, simply select Troubleshoot.
Next select Advanced options.
Next you select UEFI Firmware Settings.
Note: If you don't see the UEFI Firmware Settings tile here, your computer doesn't use UEFI. You'll need to access the BIOS in the traditional way.
Click the Restart button. Your system will restart and take you to the UEFI BIOS.
Part 2: Disable UEFI Secure Boot on the computer to boot in a traditional way next time
Once you're in the UEFI utility screen, move to Boot menu on the top of the screen and here you can see the option to turn off secure boot in Windows 8/ 8.1. Use the arrow key to go to Secure Boot option and then press enter to select it. Use + or – to set Seucre Boot to Disable, and set Legacy Support to Enable, so that you will boot the computer in a traditional way.
Note: Depending on the motherboard's BIOS/ UEFI firmware, the "Secure Boot" option may be listed under the "Boot", "Security", or "Authentication" menu.
HP Secure Boot
ASUS Secure Boot
Acer Secure Boot
ASRock Secure Boot
Next, press F10 to save the changes and exit the UEFI settings.
That's for how to disable secure boot in Windows 8/ 8.1. Note that if you are Arm based Windows RT PCs and devices users, you can't disable secure boot because these devices have a locked boot loader.