Would be interesting to hear about your problems with Windows 10. I have upgraded countless computers to 10 with no issues that couldn't be resolved. In nearly 100% it was missing or wrong hardware drivers.
If you, however, would like to go back to Windows 7, the safest way would be to use your recovery media (assuming that you created those back in time?). If you don't have them, get yourself a Windows 7 installation media and use your product key from the side / bottom / back of the computer case (assuming that your computer originally shipped with Windows 7).
Before you start, make sure you downloaded all hardware drivers to a USB stick or other media. You probably will need those after a fresh install of Windows 7.