If you have recently upgraded to Windows 10 from Windows 8.1 and would like to go back to previous operating system, Windows 10 provides a recovery option for that. Unfortunately, this revert option from Windows 8.1 to Windows 10 is available for 30 days only.
Follow these steps.
Search (WIN+S) "Settings" in your Windows 10 and open Settings dialog.
Select "Update & Security" option. See below:
Select "Recovery" option. In the right side bar, you will see an option to "Go back to Windows 8.1" or "go back to an earlier Build" depending on your previous operating system install.
Select that option and follow the steps.
If you're reverting back from Windows 8.1, the wizard will ask you for a reason and show you other options. Just follow the steps and you will be good to go.
Note: Your computer must be connected to the Internet.