Thanks for contacting us and choosing products!
Product top sale is auto get product sale from WooCommerce function.
You can’t change it manually, it will automatically display by WooCommerce.
Now if you want to change it you need edit query to get product in:
You can change:
$product_ids_on_sale = wc_get_product_ids_on_sale();
$product_ids_on_sale = array(1,2); // Change 1,2 to product IDs
You can do it via child theme follow this topic:
How to customize WordPress theme using Child theme
Hope this helps!