Thanks for asking.
If you would like to customize 404 page then I suggest you to Custom 404 plugin that can be installed from X > Validation > Extensions. To get started with the plugin, please take a look at following article:
However, if you don't want to use plugin then you have the option to make the necessary changes in _content-404.php file under /wp-content/themes/x/framework/views/global/
Before making any changes in theme files, I suggest you to install and setup child theme. You can use following resources to download and setup child theme.
Download Child theme from following source: https://theme.co/apex/child-themes
Please take a look at following article to setup child theme: