If go back isn't available
Check to see if you can restore your PC to factory settings. This will reinstall the version of Windows that came with your PC and remove personal files, apps and drivers you installed, and any changes you made to settings. Go to Settings > Update & security > Recovery > Reset this PC > Get started and look for Restore factory settings.
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.