To change your password, select "Reset Password" under "My details", fill in the old password and then fill in the new password twice. If you lose your password in the future, you can press the "Forgot my password" button and insert your work- or private email (provided that you have added your private email address in the portal). You will receive an encrypted email containing new user codes.
When you are prompted to change your password, use the one-time generated password as your "current" password. You can then create a new password that meets all the requirements.