If you can't remember your password, then go to the login page here and click on "Request new password" at the top right hand corner of the page. You will be taken to a page where you can enter your email address or your username (you can try several guesses until you get it right!) and the system will send you a new one.
If you can already log in and you would like to change your password to something easier to remember, then look for the block that has your name in the header in the left column. Click on "my account & groups." Click "edit" at the right top of the page, and enter your new password twice, once in each of the two "Password" boxes.
Then scroll down to the bottom of the page and click the "Submit" button.