Hi,
You have to do a template override, there is some documentation about it here:
www.hikashop.com/fr/support/documentatio...tation.html#override
For the quantity input:
You need to create the file templates/YOUR_TEMPLATE/html/hikashop_button.php. We invite you to look at the file administrator/components/com_hikashop/helpers/cart.php for the default code of the function you will define in it. In that file, you will be able to define the function:
hikashop_quantity_render($html,$i,$max_quantity,$min_quantity) => The $html variable will contain the HTML of the "add to cart" button, the $i variable will contain a unique int which will be different for each quantity input on the page, the $max_quantity and $min_quantity will contain the min and max quantity possible for the product. You need to return the whole HTML at the end of your function.
Also, please note that it only works if the option for "Display the quantity field on the product page" is set to "Ajax Input" in the configuration of HikaShop.