Create a new admin account

  1. Hold +S on startup
  2. mount -uw / (fsck -fy is not needed)
  3. rm /var/db/.AppleSetupDone
  4. reboot
  5. Go through the steps of creating a new account. You can leave all fields in the personal information step blank.
  6. After logging in to the new account, go to the Users & Groups preference pane
  7. Select the old account, press the Reset Password... button, and enter a new password