Hi SunflowerGUY,
In order to change your email password you will need to log into Telstra's My Account page and make the changes from there.
Under the column Plans & Usage there will be a box which says 'X Mailboxes, Manage your email accounts' where X= number of mailboxes you have. When you click on that box you will be taken to the page which will allow you to change you password.
- Ben