Update query for Drupal 8

update-term.php

/**
 * Update taxonomy term.
 */
function _zk_general_test() {
  $query = \Drupal::database()->update('taxonomy_term_field_data');
  $query->fields([
    'description__format' => 'plain_text',
  ]);
  $query->condition('vid', 'sectors');
  $query->execute();
}