Login to your Extra account and go to the "My Details" tab. Scroll to the bottom of the page, click "Change Password" and follow the instructions on screen. From here you will be able to change your password at any time.
If you have forgotten your password you can reset it securely. Go to the "Login" section of the site, click on "Login/Manage Booking", click on the "Forgotten Password?" link, enter your email address and instructions on how to reset your password will be sent to you.