You can find the password fields in the Account Information (m) page.

While you are changing the password please make sure that you have defined secret question and the related secret answer on the same page, and your birth date in the Own Information (m) page.

These settings will become important should you forget your password and a replacement password cannot be sent to the private email address defined for your account (that's why you should keep the private email address up to date). In a case like this a replacement password can be sent to an email address you choose, but only if you have defined your birth date, the secret question and answer, and remember the answer correnctly.