Hi Rosy…. if you are installing Preference for the first time, you need to unzip the downloaded file first because inside that are some other things, including another zip file which is the actual theme you install.
If you already have this installed and installing an update, first always backup your theme files (copy) and save somewhere. Download the new updated theme, unzip the file to your desktop, then do the same for the actual theme file inside. Using your FTP program, manually upload the contents of the theme folder to replace the existing theme files.