I'm assuming this was an upgrade install? If you've had Windows 10 installed for 30 days or less there is an option to roll back to Windows 8.1. Otherwise I believe you will have to format the hard drive and install Windows 8 all over again.
If you have a computer that came with Windows 10, you will have to purchase a copy of Windows 8 and do a clean install.
Win 10 – Revert Back to Win 7 or Win 8.1
Microsoft has made it surprisingly easy to return your PC to Win 7 or Win 8.1, as long as you do it within one month of upgrading to Windows 10.
A file stored on the C:\ drive called “Windows.old” contains the files needed to return to your previous operating system. After a month, that file is automatically deleted to save hard disk space.
To uninstall Win 10 within the first month perform the following steps.
1. Click on the Win 10 icon in the lower left hand corner of your monitor/screen to get to the Win 10 START MENU.
2. Click on SETTINGS.
3. From the resulting Settings Window choose UPDATE and SECURITY.
4. Click on RECOVERY. You’ll be given the option to “go back” to Win 7 or Win 8.1 depending on what you had previously.
5. Click GET STARTED. Your previous version of Windows will then be restored and Windows 10 will be gone.
If you wait longer than a month you can still revert back to Win 7 or Win 8.1 but you’ll have to do it the tedious way by using your Win 7 or 8 Installation Disks or by downloading a copy of your previous Operating System from Microsoft. Either method wipes out any data or programs you’ve accumulated since you installed Win 7/8 or purchased your computer.
To download your old operating system you’ll need the “Product Key” to prove to Microsoft that you’re entitled to do so. The product key will be on a Certificate of Authenticity sticker on the outside of the PC or in the CD/DVD case that the Windows software came in. For help finding it see tinyurl.com/nch3av6. For details on downloading old versions of Windows see tinyurl.com/knfxe3b for Win 7, or tinyurl.com/bkpy97u for Win 8.1.
There is a time limit on how long you have to make a decision to return. This limit depends on which version of Win 10 you currently have.
1) if you have version 1511, you have 30 days to return to your previous OS
2) if you have version 1607, you have 10 days to return to your previous OS
So if either of the above apply to you, then click start > click settings > click update & security > click recovery, then continue on to do a recovery
If neither of the above is applicable to you, then get your WIndows 8.1 software and reinstall it using your license (product key)
You can read this link and it explains it perhaps better than I did
you dont…. why would you want to if anything 10 is 1000X better than 8.1.