Changing CiviCRM events partcipant status
Normally it would be a civicrm user/ administrator that would update participant details after an event. (Rather than the participant themselves).

As Pete said you can configure a Drupal Webform to update participation details.

The answers to this question How do I get Retrospective feedback for an event using a Webform (participant Custom Data)?

Shows how someone else has configured a webform, and the webform url (including event id and contact id) for use in a schedued reminder.