Log in to your account by entering your user name and password in the Login Form visible on the right side   of most pages on this website. Once you are logged in, you will notice that a User Menu appears on the right  column. Click on the Edit/Update User Details link.

You can now change any details, including your user name and password.

Click on the Update button before exiting this screen.

Note: Leave the Password and Verify Password rows blank unless you wish to actually change your password. Those rows are typically blank for security reasons.