How do I change the password on my computer?
Thanks!

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If you know your password and just want to change it, you can go through the system preferences and go to the Accounts pane. There you’ll find the change password preference. However, if you do not remember your password and need to reset it, then you’ll have to boot from an install DVD of Leopard and choose the password reset utility from the DVD.

More here:
Mac OS X: Changing or resetting an account password