How to make a dynamic and easily customizable website

A client of mine wants me to develop a website in such a way that can be changed easily . . .

He wants me to give make a GUI desktop application that will allow him to make changes as and when required. My client does not know anything about coding and wants to customize his website by HIMSELF without contacting me for every change like the heading displayed and the images on the website.

Now that's where the problem is. I cant figure out how can I make a GUI app that overwrites a active website [HTML file] that's already hosted in my web server. I'm pretty sure that this is very tedious to make practically but also believe that there must be some way to achieve it.

Please suggest me the best way to do this. . .