If you need to modify functionality, instead of appearance, you will often need to insert custom PHP code in to your starter child theme. This code should almost always be placed in your child theme’s functions.phpfile. Inside this file, you’ll see a line at the bottom that says this:
/** * Add your customizations below this line. */
This is where you add your custom PHP code.
To learn more about modifying files, please read Editing Files on the WordPress Codex.