Forget my macbook air startup password

Question

I changed my Macbook Air startup password but somehow forgot the new password. How can I turn on my Macbook Air?

Answer 1

If the user account is associated with an Apple ID, and you know the Apple ID password, then maybe the Apple ID can be used to reset your user account password.
Otherwise*, start up in Recovery mode. When the OS X Utilities window appears, select
          Utilities ▹ Terminal
from the menu bar at the top of the screen—not from any of the items in the OS X Utilities window.
In the window that opens, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window opens. Close the Terminal window to get it out of the way.
Select your startup volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Follow the prompts to reset the password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.
Select
           ▹ Restart
from the menu bar.
You should now be able to log in with the new password, but your Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it.
*Note: If you've activated FileVault, this procedure doesn't apply. Follow instead these instructions.