Has your email address changed? Is there a typo in your username? Do you just want to have a custom username? PIEmatrix gives you an easy way to change your username to whatever you want (as long as somebody else didn't beat you to it). After you've logged in, click on the Settings button in the upper right hand corner (click #1). Once there, go to the Personal Setup tab and click Edit to edit your profile (click #2). You'll notice there's a little lock next to where your username is. Click it and you will be able to change your username (click #3).
You can also change your email address from this page. Changing your email address will not automatically change your username. The username and email address can be different from each other. By default for new users, PIEmatrix makes them the same.
Account Administrators can also change email addresses by editing a person. They cannot change your username though.
For any questions, please contact me at email@example.com.