When a computer is upgraded from Windows 7 SP1 or Windows 8.1 to Windows 10, the previous version of Windows is retained on the hard drive for approximately 28 days. This feature allows a user to revert back to the previous version of Windows for any reason. After 10 days (30 days in versions of Windows 10 prior to Anniversary Edition) the old version of Windows is removed to free up space on the hard drive.
If your computer is unplugged, be sure to plug it into wall power and click Next.
Click Start , then type "recovery".
Select Recovery options (System Setting).
Under Recovery, select Go back to Windows [X], where [X] is the previous version of Windows.
Select a reason for going back, then click Next.
Click Go back to Windows [X].
Follow the on-screen instructions to complete the rollback.
When the rollback process is complete, the computer will boot into your previous version of Windows. If you used a password to log into your previous version of Windows, you will need to know it to log in.
Here are some recommended articles related to this topic that might be of interest to you.