/**
 * Implements hook_ENTITY_TYPE_update().
 */
function {{ machine_name }}_ENTITY_TYPE_update(Drupal\Core\Entity\EntityInterface $entity) {
  // Update the entity's entry in a fictional table of this type of entity.
  db_update('example_entity')
    ->fields([
      'updated' => REQUEST_TIME,
    ])
    ->condition('id', $entity->id())
    ->execute();
}