The related products gallery from Wix Stores is a add on from the app itself.
https://support.wix.com/en/article/adding-a-related-products-gallery-in-wix-stores
You can’t change it yourself, which is clearly stated on the support page about it linked above.
Note: It is not possible to select the products that appear in the Related Products gallery.
If you want to make up your own then look at the tutorial here.
https://support.wix.com/en/article/corvid-tutorial-adding-a-related-products-area-to-a-wix-store-product-page