1. Turn off your Mac.
2. Press the power button whilst holding Command + R.
3. The Mac will boot into Recovery Mode – when you see the load bar appear you can let go of the keys.
4. Go to Utilities > Terminal.
5. Enter ‘resetpassword’ and press the Return key.
6. Select the main hard drive.
7. Select the User Account (the account you’d like to change).
8. Enter a new password and create a password hint.
9. Click Save – a warning will appear that the Keychain Password hasn’t changed. Click OK.
10. Shut Down your Mac and start it up again. Now you can log in using the new password.