A Child Theme is used if you want to make changes to CSS styles or functionality of the theme and don’t loose the possibility to receive updates for the main theme.
You can install Child Theme in the same way as the main theme (Installing the Theme). It comes with blank files so you should add your custom code to make changes appear on the website.
In case you want to edit a template file from the main theme, we recommend you to make a copy from template files and add them to the child theme. In this way, you don’t have to worry that on theme’s updates your changes will be lost.