For security reasons it is recommended that you change the password you are automatically issued.
Once you have logged in, click on either your username in the top right corner or on the "Team" menu and select "Change user information".
Scroll down to the bottom. Fill in your old password and your new password (it must contain at least one numeral and be at least 6 characters long) and click "Update".
Note that some web browsers, such as Safari, have a tendency to change your email address to your username on this form - this causes an error message when you click "Update". If this occurs reload the page and fill out the form again.