The password you use to log into your computer can be changed by you. You will need to know the existing password, and have access to the computer itself. Changing the password, propigates back to the Centennial account servers as well. It will change your mail password, and other services to match.
The process, is to open System Preferences (Under the Apple Menu). Click on "Users and Groups". Click the "Change Password" button. You should see the drop down with fields for your old password, your new password, a verify password space and a place for your password hint. Fill out these fields, and click "Change password". You may receive an update keychains note that you will have to approve as well once finished.