Here's how you can download the Windows 8 ISO from Microsoft.
1. Visit the Windows Upgrade page on Microsoft's site. 2. Click the Install Windows 8 button. 3. Your file will begin to download. 4. Run the program on your computer. 5. Type in your Windows 8 product key.
For more, see the original article at the link below.
How to re-download the Windows 8 ISO | Ghacks