The simple answer is that you can't do it yourself. Only Site Admins can change user names.
Obviously we prefer to keep name changes to the minimum as it just creates extra work for us, as well as confusion for other forum members. However, sometimes people do have good reason to request a change. So if this applies to you, then please post here and one of the Admins will deal with your request.
If you've had your user name changed, you simply log in with the new name and old password.
If you have forgotten your password, you can request a new one to be sent to you at the logging in stage. This is an automated system which sends an email to the email address in your profile. (Your password is encrypted in our database so we don't know what it is.)