Login on your account and go into Uppfæra reikningsupplýsingar (by clicking on your name in the top right corner) and choose Breyta aðgangsorði, write your current password and confirm a new one. When you change your password, be aware that all your other devices automatically will be logged out within 24 hours, and you may be asked to log in again within those next 24 hours.
Forgot your password? You can reset your password here. Fill in your email/username and wait for a recovery email that will be sent to you automatically.