Update field for custom entity

function api_json_update_8001() {
  $field_storage_definition = BaseFieldDefinition::create('string')
    ->setLabel(t('migrate_id'))
    ->setRevisionable(FALSE);

  \Drupal::entityDefinitionUpdateManager()
    ->installFieldStorageDefinition('migrate_id', 'api_json', 'api_json', $field_storage_definition);
}