Hi,

The image shown on the main slider is a featured image of the post. Just go to the post you want to change the image, replace the featured image on the right side.

If you want to change the post display on the main slider, please go to Appearance → Customize → Featured Slider → General, scroll down to Select Featured Category, or enter the post tag to Fill List Featured Tags for Featured Slider area.

Regards,
PenciDesign.