If you lost or forgot your password or your username, you can click on the link Request new password which you can find below the username and password field (on the frontpage). If you click this link when already logged in, you do not have to fill in your email address.
Next you can fill in the email address you signed up for on the website and hit the "E-mail new password" button. An email will be send to you containing your loginname and a one-time link that takes you to the "Reset password" page that gives you a bit of explanation. Click on the "Log in" button and on the next page change the password following the password policy rules shown (in grey below). Each time you meet one of the password rules it disappears until they are all gone. Repeat the same password for validation and hit the "Save" button at the bottom of the page. Next time use the new password.