The first slide in the home page is the "Carousel Widget". You can find it by following this path:
WP Backend >Appearance >Widgets >Carousel >(WPL) Carousel
If you would like to show your desire property in the "Carousel Widget", you will need to enter the property listing ID in the "Carousel Widget". Please see the "propertylistingid.png" that is attached.
If you would like to have the flex slider, this is different. You can find the "flex slider" by following this path:
WP Backend>WPL28 Setting > Flex slider
Upload your image and use the flex slider short code "[wpl_fs]" to generate the flex slider in your desired page.
Please see the "flexslider.png".