Windows 8 Enterprise & Windows 8 Enterprise Evaluation users will need to download the Windows 8.1 Preview ISO files following this link.

To start off, visit here and download both the Windows 8 and Windows 8.1 setup installers. Once completed, launch the Windows 8 installer and enter the product key upon prompting.

Windows 8 users who want to upgrade to Windows 8.1 without having to go through the Windows Store can use this trick to download the ISO with the Windows 8 product key.