How to Install the Windows 8.1 Preview
When you see the Open File – Security Warning dialog box, click the Open button to continue.
After you click the Open button, the installation will continue. Once the update has been installed, you'll be prompted to restart your PC. When you're system has restarted, click the Store icon to open the Windows Store. At this point you'll see a prompt offering the Windows 8.1 Preview and you should click the Upgrade button to install the Preview.
If you receive a message that tells you that the update does not apply to your system or if the Windows Store doesn't offer you the option to install the Windows 8.1 Preview, you can try a more advanced option of downloading the iso files, burning a disc, and installing the Windows 8.1 Preview from that disc.
You can find out more about this option at the Download Windows 8.1 Preview page.