To start showing excerpts instead of full posts you need to insert the ‘more tag’ into each of the posts (just like in the demo content). This is native WP functionality. Here is how to use the ‘more tag’: https://en.support.wordpress.com/more-tag.
As for the layout, if you’ve followed the installation steps in the User Guide (Documentation/User Guide.pdf in the archive you got from ThemeForest), then you now have a ‘Theme Options’ item in your WordPress Admin Panel. From there you can control the layout settings. Looks like this: