Here’s a reference to the pre_get_posts() function in the WP Codex:
https://codex.www.ads-software.com/Plugin_API/Action_Reference/pre_get_posts#Show_Only_One_Category_on_Home_Page.
You would add that function to a child theme functions.php file. If you don’t already have a child theme, creating one with Hueman is pretty easy:
1. In Theme Options click the Help tab in the upper righthand corner.
2. One of the options in the list is to download a sample child theme. This downloads the theme zip file to your local computer.
3. Install the new theme in your Admin panel by selecting Add New > Upload Theme > Choose File, then select the zip file you downloaded.
4. Activate the child theme.
You’ll now have a pre-configured child theme with a style.css file and functions.php file to which you can add customizations.