Hello,

Please read this tutorial, it will show you how to upgrade.

http://support.themely.com/knowledgebase/how-to-upgrade-integral-lite-free-to-pro-version/

If you are using a child theme like Elemental, make sure to install and setup Integral Pro first, then download and install Elemental Pro child theme.

You can find the download link in Appearance > Setup Integral > Child Themes tab.