Possible Duplicate:
When my game is compiled into exe. How do I update my games. The same as online games, with a small file to update. Thank you
I think you're referring to patches, and the short answer is: you don't. Unity doesn't handle patches for your games at all. You would have to use a third party tool (which we can't help you with), or code it yourself (which is extremely difficult).
Publish the game to steam and use it, Steam is a widely used platform by gamers and you can easily update your game with, I think.