Appearence | Themes | Customize | Slider
I had the same problem just after the import of the demo: I wanted to keep just the first slide and remove the second one, but it seemed not possible.
But it actually was very easy to do: just load another image to replace the one you want to remove. After that in the slider panel there will be the “remove image” button. Remove the image, clear the other fields related to that slide and that’s it.