For those in need of an official Windows 10 ISO file, Microsoft has released a very simple tool named Media Creation Tool to allow you to download Windows 10 ISO images, or to create Windows 10 DVD disc or USB flash media, for clean installation.
No product key is required to download the Windows 10 ISO images, or to create DVD or USB media trough the Media Creation Tool. However, a valid Windows product key is required when directly installing Windows 10 for the first time on a device.
Media Creation Tool
To use the Windows 10 Media Creation Tool, download Windows 10 Media Creation Tool from microsoft.com and run it.
In the “Windows 10 Setup” window, choose Upgrade this PC now if you want to upgrade Windows 7 or Windows 8.1 to Windows 10, and hit Next. To get Windows 10 ISO images, or to create a Windows 10 DVD disc or USB flash drive, choose Create installation media for another PC, and hit Next. We will use the second option.
On the next screen you can choose the Language, Edition and Architecture of Windows 10 that you want to download. Make sure that the selection corresponds to the license or product key you purchased.
Next, we can select either to create a USB flash drive or ISO file as the media for Windows 10 OS downloaded. If you want to burn a DVD disc, choose ISO file. Hit Next when done.
We have to choose a folder location to store the Windows 10 ISO image. Once we do that, the download will begin.
You can now locate the Windows 10 ISO, burn it, and then use it to install or upgrade any PC, by simply running the setup.exe.