http://windows.microsoft.com/zh-tw/windows/preview-faq

If you want to go back to your previous operating system

You'll need to reinstall your version of Windows from the recovery or installation media that came with your PC (typically DVD media). If you don't have recovery media:

For Windows 7 or Windows Vista: Before you update, you might be able to create recovery media from a recovery partition on your PC using software provided by your PC manufacturer. Check the support section of your PC manufacturer's website for more info.

For Windows 8.1 or Windows 8: You might be able to create a USB recovery drive. For more info, see Create a USB recovery drive. If you didn’t create a USB recovery drive before you installed the preview, you can create installation media for Windows 8.1.