You need to download Windows 10 from here
use the media creation tool to make a bootable USB flash disk and then clean install Windows 10 from there.

if you recently switched from Windows 10 home stable to insider build, you might be able to switch back from Windows settings.
open settings => update and security => recovery => go back to the previous version of Windows 10

if the option is unavailable, then use the first method I mentioned above.
hope that helps