How to split project resources into different files

Hello, I am developing game with Unity free. I am searching a way to split resources (or project or whatever) to different files so later I can write game downloader which will update game (change some game files) to new version. The reason I want to split files is that I don't want it to download everything over again even if there is only a small change (so I want only changed file to be downloaded again).

Thank you in advance.