I have forgotten my administrator password on my Macbook air. Can anyone please tell me how to reset it so I can upgrade and download

If the user account is associated with an Apple ID, and you know the Apple ID password, then maybe theApple ID can be used to reset your user account password. In OS X 10.10 and later, this option also works with FileVault, but only if you enabled it when you activated FileVault. It's not retroactive. Otherwise, see below.

Note: If you've activated FileVault, this procedure doesn't apply. Follow instead these instructions.

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:

resetp

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 the startup volume ("Macintosh HD," unless you gave it a different name) if not already selected. You won't be able to do this if FileVault is active.

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 the Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as the login password), there's no way to recover it.