Here’s how to fix it.
NOTE: THIS IS NOT INTENDED FOR MALICIOUS PURPOSES AND I CANNOT BE HELD LIABLE FOR ANY DAMAGE DONE TO YOUR COMPUTER IF THIS IS DONE INCORRECTLY.
What this does is creates a new admin account where you can change the old admin accounts password.
Turn off your computer and turn it back on. Right at the startup chime, hold down command S. This will boot the computer into a command line interface.
Once it is done settling itself type in:
mount -uw / (<–there is a space between -uw and /)
shutdown -h now
When you turn your computer back on, it will take you through setup like you just turned on your computer for the first time. It will play the intro, ask you for a language and keyboard layout, and ask you to register.
It will also ask you for a password, so put in a password that this time you will remember!
Now even though you might get worried that you have accidentally wiped your drive and reinstalled an operating system, don’t fret. Once you get into the account go to system preferences and click on accounts. Here, you should see your other admin account listed. Click on that account and click reset password. Here, you can type in your new password. Click OK and log out. You should see both your new admin account and your old one listed. then click on your old one and type in the password you reset it to, then you should be able to log in and access your files.
Once you have done this, open system preferences, click on accounts, and click on the new admin account you had just created. Then, click the -.
Tada! You just reset your admin password without knowing the old one.