<?php

$menu = \Drupal::entityTypeManager()->getStorage('menu_link_content')->loadByProperties(
    ['menu_name' => 'mon_menu']
);

$menuLinkManager = \Drupal::service('plugin.manager.menu.link');

foreach ($menu as $menu_item) {
    $definitions = $menu_item->getPluginDefinition();

    // Update du tableau definitions
     $definitions['options']['attributes']['class'] = 'sitemap';

    // sauvegarde
    ?????
}