Once a Ustream account is created, it is not possible to change the username associated with it. If you decide you want to change your username, you must close your current account (or just open a new one) and register a new one with the desired username.
Please note that even if you delete your account, the username that was associated with that account is no longer accessible.
1. Login to your account at Ustream.tv
2. Hover over your username in the top right and choose Dashboard from the drop down
3. Under the "Account" section, select "Settings"
4. Scroll down to the bottom of the page and click on the red "Delete account" button to close the account.