How to change Textures after build the game?

The solutions are both possible, but implementing the second one is much more difficult than the first (no GUI, no scripting, etc)

If you really want the second solution, you will have to make a file browser (some are free over the web). For a custom one see the .NET File and Directory classes :

https://msdn.microsoft.com/en-us/library/system.io.file(v=vs.110).aspx https://msdn.microsoft.com/en-us/library/system.io.directory(v=vs.110).aspx

Then, you could simply copy the selected file by the user with the appropriate .NET function :

https://msdn.microsoft.com/en-us/library/system.io.file.copy(v=vs.110).aspx

You won't have to "load" a texture, but simply copy it in the appropriate folder (inside the Resources folder : http://docs.unity3d.com/ScriptReference/Resources.html) with the appropriate name.

I think #1 is a better solution, saves you from having to create different UI components in the game. Simply load and atlas textures when the game boots by checking a folder for files by name. If the file matches a name for each file, then it simply replaces it. There's a number of basic methods for doing that so I won't go into them; I would simply say the first one is the better option.