Microsoft’s Upgrade Windows with only a product key website has two Windows download / upgrade tools: one for Windows 8, and another for Windows 8.1. Both tools require valid Windows key to download installation setup files. After that you can choose whether to upgrade Windows immediately or just create a bootable DVD or ISO file.

Windows 8.1 download tools requires Windows 8.1 license key and will not accept Windows 8 key. This doesn’t make much sense as Windows 8.1 is a free upgrade for Windows 8 users. Luckily there is a little trick that allows to download 8.1 setup using Windows 8 serial key.

• First download and run Windows 8 upgrade tool. Provide valid Windows 8 license key and start downloading. When download reaches 1 or 2%, close the application.
• Now download and run Windows 8.1 upgrade tool. Follow the same steps, but this time it should start downloading without asking for Windows 8.1 licence key.
• Let download to complete and follow on-screen instructions to create a bootable .ISO file.