How to replace "Enter quantity" text for Production variation?
I too had the same problem. Because this text is hardcoded and there is no configuration for updating the text.

Ways to change the text


Use String Overrides module.
Do a form_alter and change the text from Add to Cart to Buy Now


Form Alter for Enter quantity 

function module_name_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'commerce_cart_add_to_cart_form') {
    $form['quantity']['#title'] = t('Enter your Quantities');
  }
}


Hope this code helps.