..

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:

http://www.macworld.co.uk/ipad-iphone/news/?newsid=3463233&olo=email

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's running Mac OS X 10.7 or newer, restart with the Command and R keys held down, open the Terminal, and use the resetpassword command:

https://discussions.apple.com/docs/DOC-4101