<?php
// Update (reverse) order of field's items.
  $nid = 0;
  $field_name = '';
  if (!empty($nid) && !empty($field_name)) {
    /* @var \Drupal\node\NodeInterface $node */
    $node = \Drupal::entityTypeManager()->getStorage('node')->load($nid);
    $list = $node->get($field_name)->getValue();
    $node->get($field_name)->setValue(array_reverse($list));
    $node->save();
  }