It is (sort of) possible to actually edit the displayed content inside the help screens:
add_action('admin_head', 'my_function');
function my_function(){
$help="<p>This is my new content!</p>";
add_contextual_help('plugins', $help);
}
Doing so will replace the content inside the top part of the help dropdown with whatever you put in $help.
According to the source, to determine which page you want to change the help content for, you change the first parameter – here’s what it says about that:
The handle for the screen to add help to. This is usually the hook name returned by the add_*_page() functions.
It’s not exactly polished though… It looks like this section of the wordpress core could use a little work.
If you want to see that function in the source, here’s the link: https://xref.yoast.com/trunk/nav.html?wp-admin/includes/template.php.source.html#l3596
It looks like you may be able to do something by calling the global $_wp_contextual_help and messing with it, but I haven’t tried anything with that yet.