Log on as a user with administrative privileges (usually the first user set up on the computer). If you are not an administrative user, then you shouldn’t (and can’t) change someone else’s password. If you’re using Snow Leopard (10.6) or earlier, go to the Accounts pane of System Preferences. In Lion (10.7) or later, go to the User & Groups pane of System Preferences. In either operating system, click the lock in the lower left corner of the screen and enter your login password to make changes. Then select the user you want to change and click the Reset Password button.