If you have forgotten your password or username, you can reset them using "Forgotten your login details?" under "Log in with credentials" in the menu on the right. When a new window opens, you will be offered two options for resetting: via Smart-ID or by e-mail.
Remember that new password must be at least 8 characters long, contain both uppercase and lowercase letters and numbers, and must not contain references to your name, email address or date of birth.