divi child theme
If you need to install child theme for Divi or extra theme you can download the archives right below. Child theme allows you to customize your themes and keep the changes when you reinstall/update the original (parent) theme.
Divi child theme: Divi child theme
Extra child theme: Extra child theme
When you download this file please go to the wp admin>appearance>themes and install the archive file. Then activate the installed child theme. That's it.
To get more info about child theme please refer to this article: http://www.elegantthemes.com/blog/resources/wordpress-child-theme-tutorial