目标c - .m和a文件?

I have a IPhone project and I need to change some code in it. The problem is: this project doesn't have any .m file. I have all header files (.h) and some nib files. I found a .a(?) file and when I took a look in it, it had some code inside, aparently it's some kind of compilated file. Is there any way to de-compile this file?I have a IPhone project and I need to change so