Thanks for updating thread.
If you don't want to use plugin to display ads, then you can add following code in child theme function.php file:
add_action( 'wp_head', 'add_head_script' );
<!-- Add your adsense code below this -->
You will have to install and activate the child theme for this so that you will not lose the changes you make when you update the parent theme. It is a good practice to use a child theme when building a site because you will have to add some custom stuff like this which is not native to the theme. I am sharing few resources that you can use to download and setup child theme.
You can also take a look at following tutorial for more in depth information: