If you’ve purchased from StudioPress then you would have to download it from them on your account page. Go to this link and make sure you’re logged in – http://my.studiopress.com/downloads/ and you should see all of your downloads.
The latest version is 1.3. You can open up your style.css file and look at the top to find out which version you have. If it’s not, then you should be able to visit your account page and download the latest version.