Go into the 'Site Projects' tab in Site Admin, expand the project and select the USERS table displayed in the list. Enter the following query and click the 'Execute SQL' button.

UPDATE USERS
SET US_USERNAME = 'New Username'
WHERE US_USERNAME = 'Old Username';

After you have made the changes there, be sure to change the username in the qcsiteadmin_db as well. If you are using a MS SQL Server DB, access the Microsoft SQL Server Management Studio, expand 'qcsiteadmin_db', then the 'Tables' folder. Highlight the 'td.USERS' table, click the 'New Query' button then enter the following query and click the 'Execute' button:

UPDATE USERS
SET USER_NAME = 'New Username'
WHERE USER_NAME = 'Old Username';

After you have run the previous query, you must update the users password in the 'Site Users' tab in Site Admin. Highlight the user name, then click the 'Password' button and enter the new password.

Log back into the project and click the drop down for the 'Responsible Tester' field and verify that the change has taken place. You will also see the change take place in the 'User Name' field within the 'Site Users' tab in Site Admin.