Go to Website > Theme > Files and scroll to the bottom of the page - Click next page. You will have to scroll for several pages to find the file named "home-header-back.jpg" SEE IMAGE BELOW >
Delete this file using the down arrow to the far right of the file.
Then use the orange button to "SELECT FILES TO UPLOAD"
Upload a new background image (Size 2000x945) WITH THE EXACT SAME TITLE "home-header-back.jpg"
Clear the Cache to see the changes (see above question) by going to Website > Site Settings > Clear Cache