Hi,
For the moment the generated code is directly in the HikaShop product controller.
So if you want to change that, you will need to modify the file (which is a core file so you will loose your modification next time you will update HikaShop).
File : administrator/components/com_hikashop/controllers/product.php
Code to edit
function updatecart(){
echo '<textarea style="width:100%" rows="5"><a class="hikashop_html_add_to_cart_link" href="'.HIKASHOP_LIVE.'index.php?option='.HIKASHOP_COMPONENT.'&ctrl=product&task=updatecart&quantity=1&checkout=1&product_id='.JRequest::getInt('cid').'">'.JText::_('ADD_TO_CART').'</a></textarea>';
}
This "update cart link" should be improved in the future but I can't say more for the moment.
Regards,