How to Uninstall Windows 10
How to Remove Windows 10 and Reinstall Another OS
If the Go Back option isn’t available in Settings > Update & Security > Recovery, you’ll have to work a little harder to get your old operating system back.
As noted earlier, you should first backup all of your files and personal folders. Be vigilant here; when you perform these steps you’ll either be returning your computer to factory settings or installing a clean copy of your previous operating system. There won’t be any personal data (or third party programs) on the machine after you finish; you’ll have to put that data back on yourself.
With your data backed up, decide how you’re going to perform the installation of the previous operating system. If you know there’s a partition on your computer with a factory image, you’ll use that. Unfortunately, there might not be any way to know that until you follow the steps outlined here. Otherwise (or if you aren’t sure) you’ll need to find your installation DVD or recovery DVD, or, create a USB drive that contains the installation files before you start.