Here's how you can download the free Windows 8.1 Preview from Microsoft.
1. Go to the Microsoft Windows 8.1 Preview page.
2. Scroll down to the ISO files list on the page.
3. Click the download link for your version of Windows 8.1.
4. Your download will begin.
5. Click the ISO file to start the Windows 8.1 install.
6. Click the setup.exe file.
Note that the download sizes vary between 2.7 GB and 3.8 GB, depending on which one you choose. You can see a list of What's New in Windows 8.1, and you can also read the Windows 8.1 Preview FAQ for more information.