Customizing Header Image to Slider

I see that you have a slider working lower down in the page.
There are several slider plugins available, some of them are paid.
Depending upon your theme, you may be able to configure one to work in your header, possibly in a widget area.

More than likely you are going to have to create a child theme, and then modify a copy of the header.php file to substitute the header image with the slider code (if the developer provides code samples for you to use).