To change your user name click on 'My Account' in the menu bar and then 'My Profile'.
In the 'Demographics' tab of the 'My Profile' section you will be able to change your user name to anything you choose, providing it is not already in use.
To change the password, click on 'My Account' in the main menu and then on 'Change Password' (see above). You will need to provide your current password in order to create a new one.
