There are lots of things that can be done via filter hooks, but for your case try adding the following code to your theme’s functions.php file (or via a custom plugin):
// customize the logged-in username
function sac_customize_logged_username($logged_username, $current_user) {
$logged_username = $current_user->user_firstname;
return $logged_username;
}
add_filter('sac_logged_username', 'sac_customize_logged_username', 10, 2);
You can see where the same modification is applied via this function. That way, there is no need to modify the core plugin files, and so the change will remain in place even if the plugin is updated.