How to downgrade Windows 10 and reinstall Windows 7 or Windows 8.1: Before you begin

You have 1 month to go back to Windows 7 or Windows 8.1 after installing Windows 10. So make sure you make up your mind before the option disappears.

The first step is of course to back up any information you currently have on your PC that you want to keep. Changing an operating system is a big thing, and data can often be lost along the way. You can use external hard drives, thumb drives, or some of the various online cloud storage such as OneDrive, Google Drive, Dropbox, or Tresorit, that offer lots of space for free. When you’ve safely removed any documents, video, photos, or other important data you need, you’re safe to begin. Remember that this may also take a little time, so don’t start if you have plans for the immediate future.

How to downgrade Windows 10 and reinstall Windows 7 or Windows 8.1: Using the Update & Security settings

When you install Windows 10 on a PC that already has a Windows, the old version is stored away in a folder called Windows.old. While this takes up space, it also means that you can restore the version via Windows 10 itself. To do this first open the Windows Start menu by clicking on the icon in the bottom left of the screen. Select Settings from the menu.
 How to downgrade Windows 10
Now you’ll see on option for Update & Security. Click it.
How to downgrade Windows 10
On the next page you’ll find a list of options on the left, one of which is Recovery. Click this and the main pane will display a variety of choices. The one you want is ‘Go back to Windows x’ where x will be 7 or 8.1 depending on what your computer was running. Click 'Get started' to begin. If you’re using a laptop you’ll also need to connect it to a power source or the option won’t work.
How to downgrade Windows 10
You’ll now be presented with a blue screen (no death involved) asking you why you’re downgrading? Take a moment to fill this in, as it’s a helpful tool for Microsoft in gauging the user’s experience with Windows 10. Click Next when you’re done.
 How to downgrade Windows 10
Before Windows starts the process it gives a couple more opportunities to cancel, and also reminds you that if you had a password on your previous version of Windows then you’ll require it once the process is finished. If you’re happy to proceed then just click Next, then put the kettle on.
How to downgrade Windows 10
Windows will now roll back your system to how it was before the technical preview was installed.

How to downgrade Windows 10 and reinstall Windows 7 or Windows 8.1: Tidying up

On our test machine, which had very little installed on it, the whole process took about ten minutes and was pretty much trouble free. The only thing we needed to change was the shortcut to Windows Explorer in the taskbar, which had stopped working. To fix this we simply right clicked on the icon, unpinned it from the taskbar, then searched for Windows Explorer in the Start menu, dragged it to the taskbar, and everything was working again. All of our data was intact and in the right place, and the only other reboot needed was for Windows to instal a few updates. Fine work Microsoft!  
How to downgrade Windows 10