/**
 * Implements hook_update_N().
 */
function {{ machine_name }}_update_N(&$sandbox) {
  // For non-batch updates, the signature can simply be:
  // function {{ machine_name }}_update_N() {

  // Example function body for adding a field to a database table, which does
  // not require a batch operation:
  $spec = [
    'type' => 'varchar',
    'description' => "New Col",
    'length' => 20,
    'not null' => FALSE,
  ];
  $schema = Database::getConnection()->schema();
  $schema->addField('my_table', 'newcol', $spec);

  // Example of what to do if there is an error during your update.
  if ($some_error_condition_met) {
    throw new UpdateException('Something went wrong; here is what you should do.');
  }

  // Example function body for a batch update. In this example, the values in
  // a database field are updated.
  if (!isset($sandbox['progress'])) {
    // This must be the first run. Initialize the sandbox.
    $sandbox['progress'] = 0;
    $sandbox['current_pk'] = 0;
    $sandbox['max'] = Database::getConnection()->query('SELECT COUNT([my_primary_key]) FROM {my_table}')->fetchField();
  }

  // Update in chunks of 20.
  $records = Database::getConnection()->select('my_table', 'm')
    ->fields('m', ['my_primary_key', 'other_field'])
    ->condition('my_primary_key', $sandbox['current_pk'], '>')
    ->range(0, 20)
    ->orderBy('my_primary_key', 'ASC')
    ->execute();
  foreach ($records as $record) {
    // Here, you would make an update something related to this record. In this
    // example, some text is added to the other field.
    Database::getConnection()->update('my_table')
      ->fields(['other_field' => $record->other_field . '-suffix'])
      ->condition('my_primary_key', $record->my_primary_key)
      ->execute();

    $sandbox['progress']++;
    $sandbox['current_pk'] = $record->my_primary_key;
  }

  $sandbox['#finished'] = empty($sandbox['max']) ? 1 : ($sandbox['progress'] / $sandbox['max']);

  // To display a message to the user when the update is completed, return it.
  // If you do not want to display a completion message, return nothing.
  return t('All foo bars were updated with the new suffix');
}