Use the following code snippet to add/remove the menu from the Learndash frontend dashboard.

// Use the following code snippet to add/remove the menu from the Learndash frontend dashboard.

add_filter( 'learndash_dashboard_nav_menu', 'remove_menu_items_callback' );

function remove_menu_items_callback( $menus ) {

	$url = Ld_Dashboard_Functions::instance()->ld_dashboard_get_url( 'dashboard' );

	// To remove 'Assignments' menu
	unset( $menus['instructor']['assignments'] );

	// To add menu 'Custom Menu'
	$menus['instructor']['custom-menu'] = array(
						'url'   => $url . '?tab=custom-menu',
						'icon'  => '',
						'label' => esc_html__( 'Custom Menu', 'ld-dashboard' ),
					);
	return $menus;
}