Answer: You may change your password at any time by going to your User CP. Click on "Edit Your Details" under Your Profile. Once there, click on the "Edit Email, Password, & User Name" button.
You will need to enter your current password at the top, then only enter the details you want to change - leave anything you do not wish to change blank.
Update: Changing your own User Name is not available. If you would like do so, you must contact an admin (send a PM to Rachel or Twinkle).