The Main Slider is made by the "ANG Mega Slideshow:Big slideshow" widget placed in top-a position. The settings of this widget allow you to select a specific post type to display.
To change the slider in the Demo, you should to edit the posts of specific category selected in the widget. Go to Slideshow->Slideset->big slider.
While editing the slide, leave the existing HTML code and just replace the path of pictures and text. This will keep the display of the slider such as in the demo version and will look great on mobile phones and tablets.