Hi there,
Since you upgraded your license to Pro, you will have to install the Pro theme as it is a separate theme to X.
You can download the install file from your dashboard here.
Once you have downloaded Pro, you can install it from your Themes pages of your wordpress admin.
You can find more information about how to install a Wordpress theme here.
Once you have Pro installed, go to the plugins page and deactivate Cornerstone. Now, all you have to do is to activate Pro and you do not need to separately install Cornerstone as it already comes with Pro as the Pro builder.
When you open a page, you will see an option to edit it with Pro.
Hope this helps.