Hi Atif,
You will not be able to edit it in powershell module.
Can you try http://exchangeshare.wordpress.com/2008/07/16/how-to-recreate-owa-virtual-directory-exchange-2007/?
Hope it is pretty similar to http://msexchangeteam.com/archive/2010/02/04/453946.aspx but did you try doing a resetiis after the changes were made?
- Nagesh