Installing Windows 10 using the media creation tool - Windows Help
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If you upgraded to Windows 10 on this PC by taking advantage of the free upgrade offer and successfully activated Windows 10 on this PC in the past, you won't have a Windows 10 product key, and you can skip the product key page by selecting the Skip button. Your PC will activate online automatically so long as the same edition of Windows 10 was successfully activated on this PC by using the free Windows 10 upgrade offer.
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In short, if you don’t take advantage of the free upgrade during this offer, you won’t be available to install (or reinstall) Windows 10 on the same computer in the future.
Also, if you opt for the free upgrade, you have 1 year from July 29, 2015 to install Windows 10 at your leisure.