For security reasons it is not possible to change your username on your existing profile (otherwise a member misusing the service could quickly keep changing their identity).
How to change your username
If you really want to change your username – all you need to do is close your current account and then you will be able to create a new one with a new username. If you do not close your current account the system will not allow you to create a second account.
See how to close your account here. Any problems contact me using the link at the bottom of this page.