How to Reset a Lost Admin Password on Mac OS X

Using Recovery Mode

Reboot your computer. If you can’t change your password using your Apple ID, you can use Recovery Mode to reset it. Recovery Mode needs to be accessed as your computer is starting up.
Hold .Command+R as soon as you hear the chime. Continue holding the keys until you see the loading bar appear. This will boot your Mac into Recovery Mode. It may take a while to load.

Click the “Utilities” menu and select “Terminal.” You’ll see the Utilities menu in the bar at the top of the screen.

Type reset password and press Return. This will open the Reset Password utility.

Select your hard drive. If you have multiple drives in your Mac, you’ll need to select the one that your operating system is on. This will typically be labeled “Macintosh HD.”

Select the user account you want to change the password for. Use the drop-down menu to select your user account.

Create a new admin password. Enter your new admin password twice to create it.

Enter an optional password hint. This hint can be displayed if you’re having difficulty logging in.

Click the Apple menu and select “OS X Utilities” → “Quit OS X Utilities.” Opt to restart your Mac when prompted. This will reboot your computer and apply your new password.

Log in with your new password. After rebooting, select your user account and log in with your new password.