Oh, so before a refund or replacement I better to restore the computer back to its original?

The only time you need a product key during a Windows 10 install is if you are installing on to a machine that has never, ever had Windows 10 loaded and activated on it in the past.

If the machine has had Windows 10 loaded and activated, the license will be fetched from Microsoft's servers.  Even if you install without an internet connection once the internet connection is established the license key will be fetched then.

Now, after having said the above, if we're talking about a virtually brand new system I would not try reinstalling Windows 10.  It certainly won't, and shouldn't, void any warranty but if you have something that is within the return window the less you do to it the easier it is to get a return for refund or replacement approved.

--

Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041

If your joy is derived from what society thinks of you, you're always going to be disappointed.

        ~ Madonna