how to override "add to cart" button?

I need to add word "quantity" on the product page before quantity to add to cart.
I show this on the screenshot.

If I override "quantity" view, so in the content module I see this phrase too.... But that is not needed...

I found the file where is this output located "administrator\components\com_hikashop\helpers\cart.php"... but I don't want to change it.

what should I do?

Instead of overriding the quantity view file, you should override the show_default (show before 1.5.6) view file and add your text after:
<div id="hikashop_product_quantity_main" class="hikashop_product_quantity_main">