EaseUS System GoBack Free allows Windows users to safely downgrade their Windows 10 system to Windows 7, 8 or 8.1. The tool creates an image of your old operating system to use it afterward if you decided to downgrade the latest Windows 10. Microsoft even offers Windows users with an opportunity to downgrade their computer to their previous version of operating system after an upgrade, but the opportunity lasts for only 30 days. The software works only when you run it prior to upgrading to Windows 10.
Source: https://thehackernews.com/2015/08/downgrade-windows-10-reinstall.html