If you have given Wndows 10 a fair try of over a month or did a clean install when you upgraded to Windows 10, that computer previously had WIndows 7 or 8.1 on it and you want to downgrade then it can still be done but just not as easily as downgrading within the first month.
To get back to an older version of WIndows, you’ll have to perform a clean install of Windows.
These days Microsoft offers easy downloads for Windows 7 and 8.1 ISO files.
Simply Download the Windows installation media and burn the ISO file to a disc or copy it to a USB drive using Microsoft’s Windows USB/DVD download tool.
You can then boot from disc/USB and reinstall Windows 7 or 8.1 fresh, telling it to overwrite the Windows 10 system already on your hard drive.
Before you do so make sure you have backup copies of all your important files from your WIndows 10 PC first.
You’ll have to find your PC’s product key.
On a Windows 7 PC, examine your PC for a “certificate of authenticity” sticker with a key on it. They sticker may be on the underside of your laptop, in the battery compartment under a removable battery, or even on a laptop’s power brick.
On a Windows 8 PC, you may not have to do this at all the key may be embedded in your computer’s firmware and Windows 8.1 will automatically detect it and allow you to reinstall Windows 8.1 without even asking you to enter a key.
If the downgrade is within the first month read this guide.
Source How-To Geek