Click the pencil in the request's Actions column to change the role, date, time or notes and Save (editing does not re-send the notification email). To remove it, open the same window and click Delete Request, which also discards all signups. Only requests with status Open can be deleted; a filled or cancelled one returns "You cannot delete a shift request that has already been filled or cancelled." In the current version there is no button that sets the Cancelled status, so deleting is the way to withdraw an open request.