If you have a backup of Windows 8, restore from that backup. However, if you do not, the process is a bit more tricky.
As you re-formatted the hard drive, the copy of windows 8 you were resetting has gone, you won't be able to get it back from windows 10. You will want to install a fresh copy of Windows 8. Most laptops have the Windows product key on a sticker on the laptop or in the box; ensure you have this, or use a program to find it (boot into safe mode on Windows 10 and use a program like produkey) Make sure to backup your files before you do this, and make a list of any programs you want to reinstall. Go to
http://windows.microsoft.com/en-gb/windows-8/create-reset-refresh-media and get a USB stick ready. Download the program from the website, which will put a copy of Windows 8.1 on the USB stick. Then boot up your laptop to the USB stick - you might have to change BIOS/UEFI settings to do this. Then install Windows 8.1, enter your product key - if it does not work, there should be a phone number for Microsoft - ring the number and explain your situation, they should let you activate it. Then you can re-install programs etc.
Hope this helps.