You have TWO options!

Updating by FTP

Simply login to your FTP, browse to wp-content/themes/ and move the theme folder over, clicking "Replace All" when the dialog appears.

Updating "the other way"

You need to rename the old theme folder, which you can do by FTP or by your File Browser. If you have cPanel this is a lot easier than FTP. After you rename the folder, you can upload the theme via the "Add Theme" feature of WordPress. DO NOT UPLOAD THE ENTIRE ZIP YOU DOWNLOADED, SIMPLY ZIP THE RTTHEME00 FOLDER INSIDE OF THAT ZIP AND UPLOAD IT TO WORDPRESS.

Thanks,
Landon Wilson

RT-Themes Support