where to find the php/js/css files to revise my pages content

I’ve just taken over this website project, and currently, I hope to modify the default option of a dropdown menu（actually delete the default option）. I only have a little bit of coding knowledge, please tell me where I can find the entire website’s code resources so that I can make changes on my own later, as well as how to locate the php/js/css file where the control I want to modify is situated.