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.

This is all you need to know about Windows 10 activation:
http://answers.microsoft.com/en-us/insider...2f-4b21fbd807a7

Too long to cut and paste here.