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To access the Reset Password utility, you will need to use the Terminal in the Recovery system, so first boot to the recovery partition by holding Command-R at start-up (or by holding Option and selecting "Recovery HD" from the boot menu), and then selecting Terminal from the Utilities menu.
When this is done and a Terminal window is open, simply run the command resetpassword (all lowercase) to launch the Reset Password utility.
The program will launch, but do not close the Terminal window or the program will be killed as well. It is normal to see program output messages show up in the Terminal window when opening programs in this manner, so don't worry if you see a warning or two show up there.
With the Password Reset utility now open, you can select the volume and the account, and change the password for that account.