Download ISO image of Windows 10

You'll probably see a Getting a few things ready message when you first open the Windows 10 Setup program. When that screen clears, follow these steps in order:
  1. Read through the license terms and then accept them with the Accept button.
  2. Choose Create installation media (USB flash drive, DVD, or ISO file) for another PC and then select Next.
  3. Choose the Language, Edition, and Architecture you want the ISO image for.

    If you're going to be using Windows 10 on the same computer that's running Windows 10 Setup, you can use the default options that pertain to that specific computer. Otherwise, uncheck Use the recommended options for this PC, and then edit those options yourself.

    For most users, Windows 10 Home or Windows 10 Pro is the way to go for the Edition. The N editions are designed for some special European economic zones.

    For Architecture, choosing Both is probably the smartest way to go so that you can install Windows 10 on both a 32-bit or a 64-bit computer.

    Choose Next when you're finished deciding.
  4. Choose ISO file on the Choose which media to use screen, followed by Next
  5. Decide where to have the Windows 10 ISO image stored and then select Save to immediately start the download.

Once downloaded, you'll have a legal and full version of Windows 10 in ISO format. You can then burn that ISO image to a disc to install later or use it directly with your virtual machine software if you're going that route.

You could also burn that ISO image to a USB device, but doing that using the software's built-in tool (below) will be easier.
The free upgrade to Windows 10 (from Windows 8 or Windows 7) expired on July 29, 2016, and the free Windows 10 upgrade offer extension for users of assistive technologies ended December 31, 2017. So, you'll need to have a valid product key to install Windows 10.
Buying Windows 10 is the only way to get a valid product key. Windows 10 Pro is available directly from Microsoft but Amazon sells copies, too. Windows 10 Home is the same deal: direct from Microsoft or via Amazon are best.