Hi there,
If you are using the Pro theme you should not have the Cornerstone plugin installed. Kindly make a complete backup of your website and update the theme to the latest version which for Pro is 1.2.7.
Then go to Plugins menu on the WordPress Dashboard and deactivate and delete the Cornerstone plugin. After that you can go to the page you want to edit and you will see a new builder called Pro Editor will be available for you.
In such a process you will not lose any data, but we strongly suggest that you backup your website before doing so.
Thank you.