You can. This is how the official downgrades OEMs were offering worked. Your PC may even come with such a downgradel you can find out by calling Microsoft, giving them your Windows 8 product key and asking. (or check the product description from where you got it from, it might say)
Before installing, you have to make sure the UEFI "secure boot" is disabled in the UEFI BIOS.
You'll probably lose all your installed applications, settings, user profiles and such so it's more of a fresh install than a downgrade. But that's the only way to do it without reformatting, I'm afraid.