Click the Contact Us link at the bottom and send an email to the email address on that page.
This is the e-mail address: email@example.com
If the handle cannot be changed, you could ask them to delete your porfile, and then create a new one with your desired handle.
<b>Moderator Note:</b> This is correct – your IT Knowledge Exchange “handle” or username cannot be edited or changed. If you wish more information, please contact us at mailto:firstname.lastname@example.org