You first need to boot your Mac into single user mode by booting it up while holding down the “command+s” keys.

Once your in Single User Mode type in

mount -uw /

That will mount your Mac hard drive. Now run

launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist

and now you can change the password.  Replace “username” and “password” with the username for the user and the new password.

dscl . passwd /Users/username password

Now you can reboot the machine

reboot

and log in with the new password.