Hover on the image that you want change / edit.
You will see a edit icon at the top left corner of the widget.
Click on it.
Once you click on the icon a pop-up will appear, you can drag and drop a new image or choose the image present in the content management by clicking on Gallery
After you click on a image you will be redirected to another pop-up.
Here you can click on "Select" and choose the image to be uploaded on the page or "Choose another"