The \Drupal\Core\Field\FieldTypePluginManager class defines the alter method as field_info. Modules that implement hook_field_info_alter in their .module files have the ability to modify field type definitions discovered by the manager:

/** 
 * Implements hook_field_info_alter(). 
 */ 
function mymodule_field_info_alter(&$info) { 
  $info['email']['label'] = t('E-mail address'); 
} 

The preceding alter method will change the human-readable label for the email field to E-mail address when selecting the field in the user interface.