Can I reinstall Windows 10 on my computer after upgrading?
Yes. Once you’ve upgraded to Windows 10 using the free upgrade offer, you will be able to reinstall, including a clean install, on the same device. You won’t need a product key for re-activations on the same hardware. If you make a meaningful change to your hardware, you may need to contact customer support to help with activation. You’ll also be able to create your own installation media, like a USB drive or DVD, and use that to upgrade your device or reinstall after you’ve upgraded. To learn more, click here.
I've also read on other sites that it is possible to snag a copy of the ISO if you wanted to go that route.