Try this:
• Power on or restart your Mac.
• At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
• Type "sh /etc/rc" and press Enter/Return.
• Type "passwd <username>" and replace “<username>” with the short name of the user account for which you’d like to reset the password.
• Type your desired password and press Enter. It won’t show itself on the screen, so be careful what you type. You will most likely have to enter it again to confirm.
•Type "reboot" and press Enter.