If you have activated the Windows 8 / 8.1 key at least once on your machine then you may reinstall / clean install your Windows 10 copy as many time as you want as long as you does not change your motherboard, (as it is deemed as a new machine by Microsoft).

*However changing motherboard doesn't mean your key is void, simply call Microsoft customer service, they will guide you, provided you own a retail product key (not an OEM key). I have done this with two different motherboards before.

Download the official Windows 10 .iso here
http://www.microsoft.com/en-us/software-download/windows10

You will be prompted with product key boxes during pre-installation process, but you can simply skip it. Your Windows 10 will be activated once you're online.

*Since your product info is already on Microsoft server (from your previous Windows 8.1 activation) you don't have to worry about the product key.