Hello
If I am understanding your problem correctly you want to revert back to windows 8.1? The first thing you have to do is enter the BIOS when the computer starts up. You can usually access it by holding Ctrl or F2. Once inside the BIOS, look for start up options. You want to change the boot priority of your drives. Move the CD-ROM to the top, making it the first device the computer will want to boot from. Save and exit the BIOS. The computer should ask you if you want to boot from the disk if it is set up correctly. It tells you to press any key to boot from disc. Once you get this working the setup is straight forward, just follow the onscreen instructions.
Hope this helps