You can do it within 30 days of installing Win10. If you navigate to the Settings Menu, and then click Updates and Security, then Recovery, you will see an option to revert to the previous version of Windows. I've done this two or three times, all of them without problems. Outside of the 30 day grace period it's back to a clean installation. Again, my experience suggests that anyone having problems with the Windows 10 upgrades, either didn't take the time to check potential compatibility issues, or had malware. Possibly a FUBAR Windows installation; and upgrade installations seldom fix those.