An administrator or other user with the correct permissions can edit user information, including usernames and passwords, with the User Edit window, but any user can edit their own username and password using this window.
To change you username, enter a new one in the User Name field. Your username must be unique.
To change your password, enter it twice in the Password and Repeat Password fields.
When you are done, click the OK button to save your changes. Or click the Cancel button if you have changed your mind.
You can open this window by going to Fusion Core → Misc → Change Your User Info.