[Theme Horse Support Team]

Hi Suyaka Fukunda

At first deactivate your previous theme clean retina pro and activate twenty eleven / twenty twelve / twenty thirteen theme which is the default wordpress theme.
Then rename the clean retina pro theme and give the new name. You are getting the installation already exists because there is already clean retina pro version.
the only install the updated new theme. After installation you can see two clean retina pro theme with same name and image. The only way to know the latest version is you just check the theme version.
Then you activate it.

Hope you got it now 🙂

Sunil