The theme has the two option i.e you can either set the banner image or the slider. If you select the Banner option then you can find the option within customizer to upload the image of your choice.
Similarly if you select the slider option then you have to set the slider by choosing the category of your choice.
In this case the image in the slider comes from the featured Image section of the individual post which you have assigned to the category that is chosen to display in the slider section. So just go to post edit screens and change the image as per your requirement.
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