Move Navigation Below Page Header

// Renders the navigation below the page header instead of before
add_action( 'after_setup_theme', function () {

   remove_action( 'generate_after_header', 'generate_add_navigation_after_header', 5 );
   add_action( 'generate_after_header', 'generate_add_navigation_after_header', 15 );

} );

Source