Run migrate import with update option

gistfile1.txt

  $migration_id = 'xxx';
  /** @var \Drupal\migrate\Plugin\MigrationInterface $migration */
  $migration = \Drupal::service('plugin.manager.migration')
    ->createInstance($migration_id);
  $migration->getIdMap()->prepareUpdate();
  $executable = new MigrateExecutable($migration, new MigrateMessage());
  $executable->import();