I have a MacBook Air and I lost (forgot) my Admin (root) password
There are two ways. If you account is in the sudoers list, you can use this command:

sudo passwd root


Otherwise, boot into Recovery Mode 


Restart the Mac.
Right after you hear the chime, press and hold the Command and the R keys for a few seconds, then release. Soon, you'll see the Apple logo and spinning gear.
This will force the Mac to boot into OS X on the recovery partition that was created when you installed OS X 10.7 or later.
When booting is complete, you'll see a window of "OS X Utilities." You won't do anything there. Instead, you'll go to the Menu bar at the top and select Utilities > Terminal.
In the terminal window, type the following and press the Return key.

resetpassword
A new window will open and click on the volume of interest and select the desired user account in the popup. Then enter the new password twice. A password hint is optional. Click "Save."
Go back to the Apple Menu at the top. Select OS X Utilities > Quit OS X Utilities. That will prompt you to restart the Mac.


You can find the guide here to reset the root password here.