It’s actually part of the theme and not the slider. There are only two ways of replacing the curved graphic:
1. replacing he existing on in the theme’s “images” folder
2. using a child theme (more work involved)
You will need to decide what method you want to use. As I mentioned, I am planning to make some changes to no longer depend on a graphic image for the curve, instead using CSS, but will be a while before I implement that in an update.