My father had to take his laptop to a repair shop because it was originally Windows 8 and upgraded to Windows 10. But Windows 8 was deleted from the system when Windows 10 was installed and there was no system restore option to fix the problem instantly at home.
If I remember correctly, one could only uninstall the upgrade for a certain time, however, IF you look into your 'Computer' Local Disc (C\)Folder, you may see a folder called, 'Windows. old', if you can see it then you can 'roll-back' to Windows 8\8.1,BUT, do NOT interfere with the 'Windows' folder that may be next to it.
Now, If you’ve upgraded a PC to Windows 10—not performed a clean install, but an upgrade—you have an easy option that lets you revert to the last version of Windows. To access this, hit Windows+I to open the Settings app, click the “Update & security” icon, and then switch to the “Recovery” tab. You should see a “Go back to Windows 7” or “Go back to Windows 8.1” section. Click the “Get Started” button in that section to get rid of your Windows 10 install and restore your previous Windows install.
If you cannot downgrade using the above 'restore' method, then it will be necessary to use a fresh media install from a DVD that has a legitimate 'Key'.
Just place the DVD in the optical drive, then restart the computer where you will be asked to press any keyboard button to continue.