You can't update your username, but you can update your password, memorable question or answer and security questions.
First, log on to online banking. Then select your account name on the top menu bar. From the options, select 'Log on & security settings'. Here you can update your details.
When changing your security settings, be sure to choose passwords and answers that no one could guess. Memorise them and never write them down. This will help prevent others from accessing your financial information.