The only easy-safe way is to replace from the new downloaded theme folder, only the updated files. You can see in log change the files being edited by the update. Upload these into your installation and remake your changes if any of your edited files is affected.
You can find a detailed change log in Specular theme profile in ThemeForest.
To update Visual Composer to the one included in the latest theme update, just go to wp-content\plugins and delete(or rename) js_composer folder. Next, go to dashboard and you will find a message requiring you to install required plugins. Follow the steps and you’re done!
