How o I set my computer back to windows 8?

Pop in a win8 installer to the DVD drive, boot ir, choose to Install New, format the hard disk, sit back and relax.

Why 8? 7 is more stable and allows you to keep control of your PC.

Go to the main Settings menu in Windows 10, click “Recovery,” then click “Go back to Windows 8.1.” or “Go back to Windows 7” and follow the on-screen instructions.

The system will begin the process by asking why you’re rolling back to a previous version. Any answer will do,

Why would you want to??   8 is obsolete and open for hackers. 

You don't, you keep using 10 because it's better.

If it's a laptop you might be able to restore it to 8 using the hard factory reset.

The only way you could do that is to wipe out everything on your computer and download 8.