Q: How do I install a new password?

Is it your Apple ID's password that you forgot?

For that go to http://iforgot.apple.com

Is it not the same Apple ID you used to log in here?

you need to reset the password for your Apple ID if this is what is asked when trying to download apps from the app store.

Administrator Password?

Reset Password

Follow the instructions in the beige and green boxes.

http://pondini.org/OSX/Password.html

Note

You should now be able to log in with the new password, but you won't be able to unlock the Keychain. If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it. You’ll need to reset your keychain in the preferences of the Keychain Access application.

http://support.apple.com/kb/TS1544