edit to wordpress file. Keep header Navigation fixed to top. Only on desktop. Keep mobile unchanged.

I am currently building a site for a client and they have requested the top header navigation remained fixed to the top as the user scrolls down the page. I have had no luck getting this set up so I need someone to quickly edit the files starting right away. For a good developer this should only take a matter of minutes. The website is responsive but I do not wish the navigation on the mobile to be changed. I will supply ftp details etc and also images of exactly how I want it to look. Any questions please ask.