This document explains how to reset a forgotten account password in Mac OS X for version 10.7 (Lion) and 10.8 (Mountain Lion). Please note this method only works if you set an Apple ID to tie with the OS X user account.
If you are using Mac OS X 10.6 or earlier, please see Mac OS X (10.6 and before) - Resetting a Forgotten Account Password
One of several changes to Mac OS X Lion is how resetting passwords is handled. Previous versions of Mac OS X would have a password reset tool easily accessible from the "Utilities" menu which is not present in 10.7.
In order to reset an account password in Mac OS X 10.7 or 10.8, you must have access to your Apple ID username and password. The Apple ID is used for iTunes and App Store purchases and is also tied with iCloud. If you do not know your Apple ID, please contact the DoIT Help Desk for additional help as there are other methods for resetting the password.Please note that this method won't work for users with FileVault 2 protection enabled.
After incorrectly entering a user password three times at the Mac login screen, a message appears saying "If you forgot your password, you can reset it using your Apple ID", click the arrow button to bring up a "Reset Password" dialog.
Enter the Apple ID login information that is tied to the OS X Lion user account and click Reset Password.
Click OK to confirm the new keychain creation.
Enter and verify a new password, fill out the password hint field, and again click on Reset Password.
Click Continue Log In to login as the user account with the reset password
Also note that this leads to the creation of a new keychain, but the old keychain will remain intact if you do recall the old password sometime in the future.