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#11
I assume you want to reset it back to just after you got it, not to Windows 8 but to a clean install of Windows 10. If that's what you want you can Reset windows 10. This removes all your installed apps and reinstalls Windows 10 as if it has just been installed. You do have the option to keep your user files, but anything you installed that didn't come with Windows will be removed.
Reset Windows 10 | Windows 10 Tutorials
Before you do that though, you might like to try an in-place repair install. This will also re-install Windows and correct most settings that have got out of sorts. It will however let you keep all your installed apps as well as the user files.
Repair Install Windows 10 with an In-place Upgrade | Windows 10 Tutorials
Once you have the system working as you want it would be a good idea to make a system image to an external HDD. This allows you to restore the PC back to the state it was in when you made the image, should it get slow again. Most here recommend Macrium Reflect (Free).