How to change system password on mac
Click on Launchpad
In the list of dock select “System Preferences”
Click on “Users & Groups” icon
Click the lock to make the changes in the bottom of the popup window.
Click on Change Password button
Enter old password and new password
and again click on lock the changes to save button
close window .
Now you password changed successfully.
http://www.youtube.com/watch?v=c9dWU-dEYgM