To change your username and/or password, click on your name, which is always displayed in the top right hand corner of the screen.
The ‘user settings’ popup will then appear. The fourth text box down will display your existing username and will allow you to overtype with a new username.
To change your password, click the ‘change password’ tick box. This will then display the password text field. This will be blank, allowing you to enter a new password, which must be a minimum of eight characters, including text and numbers and there must be a capital letter at the start of your password.