By default featured image is showing at first on single product page. We shall show the first image of product gallery images instead of featured image. If a product have not gallery images, then featured image will display.
Open the functions.php file of your theme and add this PHP codes at end of the file.
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How do you upload the product gallery images?
Product gallery is an inbuilt feature of WooCommerce plugin. So you do need any extra code or plugin for it. You will go to your product page at your Dashboard. You will get Product Gallery meta box at right side. You can add multiple images there. See the attached file.