Resetting or changing a password:
For Snow Leopard or earlier: http://support.apple.com/kb/HT1274
For Lion or later: http://support.apple.com/kb/HT6022
This is also useful:
If it's running Mac OS X 10.6.8 or earlier, insert a Mac OS X install DVD, restart with the Option key held down, click on it, and use the Reset Password utility.
If it is the Keychain password that needs resetting:
If the computer has a firmware password set, contact Apple.
I followed the directions using my Install Disc and did change the second user password I was after. Now when I try to sign-on to that account I get a message stating -" The password was recently changed and please enter the previous password". I'm still stuck. It appears that file vault is on for that user and won't allow a change without the "Master Password".
What else can I try? Thanks.