A simple way to change the password of a user is to boot into single user mode in the mac. This requires a cmd-s upon startup. This will launch the command line.
In earlier versions you then mount the root filesystem read/write
-> mount -uw /
Then change the password.
-> passwd username
In later versions of OS X you mount the filesystem read/write then
-> launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
-> passwd username
Exit and you are done.