Mac: Change root password without old password

If you don’t have access to an administrator already, you need to acquire root access.

If you don’t have admin access, boot the computer into Single-User Mode by holding CMD+S on startup, mount the drive, and type the command:

Wait until you see a bunch of white writing scrolling by on a black screen. You can release the keys, and you are now in Single-User mode. The computer is ready to continue when you see localhost:/ root# or something similar as the very last line on the screen.

Then follow the steps to reset admin password without old password

 /sbin/fsck -fy
        This command is a 'File System Consistency Check' and works very much the same way as Disk Utility's 'Repair Disk' function.
  /sbin/mount -uw /
        The mount command gives us access to the files on the system so we can make changes.
   
 launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist

 and finally:
 passwd
        Then, enter your new root password when prompted twice. After the password has been reset, type:
       
 reboot
        this command restarts the computer.