Here I can use Windows 10 own reset and it stays with 10. On some PCs with factory recovery it goes back to what the factory issued. Here's a link about W10's own reset.
"To reset your PC
Swipe in from the right edge of the screen, tap Settings, and then tap Change PC settings. ...
Tap or click Update and recovery, and then tap or click Recovery.
Under Remove everything and reinstall Windows, tap or click Get started.
Follow the instructions on the screen."
Now there are some that want to write you need to clear unused space which will take some time. After the W10 recovery, I think I'd use CCleaner's clear blank space feature.
Noted at https://www.piriform.com/docs/ccleaner/using-ccleaner/wiping-free-disk-space
My father has a desktop and he upgraded it to Windows 10.
Now we want to sell it but are now wondering, if we reset it to factory defaults will it go back to Windows 7 or will it stay on Windows 10 ![]()

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