How to reset Mac password

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Apple includes a tool with Mac OS X to replace a Mac's password. This is the best option if you have completely forgotten your password. Follow these steps:

Turn off your Mac (choose Apple > Shut Down).

Press the power button while holding down Command-R. The Mac will boot into Recovery mode.
When you see the load bar appear you can let go of the keys. It may take a few minutes to load.

Select Disk Utility and press Continue.

Choose Utilities > Terminal.

Enter resetpassword (all one word, lowercase letters) and press Return

Select the volume containing the account (normally this will be your Main hard drive).

Choose the account to change with Select the User Account.

Enter a new password and re-enter it into the password fields.

Enter a new password hint related to the password.

Click Save.

A warning will appear that the password has changed, but not the Keychain Password. Click OK.
Click Apple > Shut Down.

Now start up the Mac. You can login using the new password

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Reference: http://www.macworld.co.uk/how-to/mac/what-do-if-youve-forgotten-your-mac-password-3594395/