How do I reset a user's password in Mac OS X?
If a user forgets their password in Mac OS X, you can use the install disc to reset ANY of the user account passwords.
Boot from the install disc (or any install disc with a version higher than the installed OS X version) by having the disc in the drive and holding down C on bootup.
Choose Reset Password from the top Utilities menu.
If a user has activated the Master Password and turned on File Vault, do NOT reset a user’s password in this fashion as this will make the data unrecoverable unless you reset the account back to the original password.