I know that because it is already downloading into a hidden C:\$Windows.~BT folder onto my machines.
Now one question that many people are asking is “Can I do the free upgrade as a clean install if I have downloaded the ISO image?”
The short answer is No.
The long answer is ….
Can I perform a clean install using the Free upgrade?
No, it will require that you are running a previous qualifying version and start the upgrade from within the qualifying version. You can initiate a clean install after completing the Upgrade.
Please be aware that you cannot use the free upgrade offer to perform a clean install on first attempt. You must first upgrade from the qualifying version of Windows 7 or Windows 8/8.1 (whether you do it through Windows Update or using the .ISO file). Ensure the upgrade is completed successfully and then ensure that it is activated. You can then proceed to do a clean install by using recovery media or using the Reset function in Start > Settings > Update & Security > Recovery > Reset this PC (Get Started).
For more information on all your Windows 10 questions, please see the following article:
This article was originally posted on http://www.winthropdc.com/blog.