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There's a setting in the Admin panel for you to change your username.
From the left window frame in the Administration Panel (http://yourforum.com/admin/), click the Permissions from the 'User Admin' area.
Enter the username and click Look up User button or click the Find a username button to list the existing usernames.
Follow the onscreen prompts.
There's also a setting that allows you to let all other users change their usernames as well.
To do this go to:
Administration Control Panel -> General Admin -> Configuration, scroll down to User and Forum Basic Settings and select YES to Allow User Name Changes.
If this is checked, users can change their username by simply going to their Profile and deleting what's written in the username box and writing in a new name for themselves.