How to Edit Fetured Image

what do you want to edit?

Various things are controlled in various ways. You will need to do it through functions.php most likely. BUt smaller things can be done through regular template files

It doesn’t exist in the sense you are thinking of

The sizes exist in functions.php

http://voodoopress.com/2011/04/woking-with-featured-image-in-wordpress/

May help you understand. You can change, or add new image sizes