I really need it for my server, so please help.

to edit terrain.png, you'll need to extract default.zip and then open it, and then you'll need a drawing program (Paint.NET, Gimp, Krita, etc.) to edit it, once you are done editing the png, you can save it and then you'll have to zip the default back to zip form and then you can drag it to the texpacks folder and use it.

 

(sorry this was not detailed im currently tired as im making this reply.)