If you forgot your password or log in, you can reset it on the log in page (My Account) by clicking the "forgot my password" or forgotton username button. It will email you over a new, temporary password that you can change at any time. This link will expire in 30 minutes but could take up to 10 minutes to be delivered to your email box.
If you just want to change your password, log into My Account. On the left side of the dashboard, you'll find a section titled "manage account". Click on 'change password'.