To go back to a previous version of Windows, select Settings > Update & security > Recovery. The period you have to go back depends on what version of Windows 10 your PC is running after the upgrade. For example, if your PC is running Windows 10 Version 1511, you have one month to go back to a previous version. If your PC is running Windows 10 Version 1607 or later, you have 10 days to go back. (To see what version of Windows 10 your PC is running after you upgrade, select the Start button, then select Settings > System > About.)
If going back isn't available, try restoring your PC to factory settings. This reinstalls the version of Windows that came with your PC and removes personal files, apps and drivers you installed, plus any changes you made to settings. Go to Settings > Update & security > Recovery > Reset this PC > Get started and locate Restore factory settings.
Or,if you have a product key for your previous version of Windows, use the media creation tool to create installation media for Windows 8.1 or Windows 7, and use it to do a clean install.