If you've forgotten your password, click on "Have you forgotten the password?" on the login screen. Enter your email address and click "Recover" to receive an email with a link (please note: it may take a while to arrive, and remember to check your junk/spam folder). The link is active for 48 hours and can only be used once; clicking it will allow you to set a new password, which you can then use to access your account. After 48 hours, the link expires, and you can click "Have you forgotten the password?" again to receive a new one.
This same procedure can be used to change your password.