How to change the 'Quantity' label in commerce-order-item-add-to-cart-form I'm trying to modify my 'Add to cart' form, changing the 'Add to cart' button text and the 'Quantity' label:

if(strpos($form_id, 'commerce_order_item_add_to_cart_form_commerce') !== false) {
    $form['actions']['submit']['#value'] = t('Book Now');
    $form['edit-quantity']['und'][0]['value']['#title'] = t('Number of days');
  }
}


The first part is working, changing the 'Add to cart' button text, but the second part is failing changing the 'Quantity' label.

Any idea how to fix it?
To change label try with:

$form['edit-quantity']['widget']['value']['#title']  = t('Number of days');


Or:

$form['edit-quantity']['widget'][0]['value']['#title']  = t('Number of days');


Use Devel to check $form['edit-quantity']['widget'] with dsm($form['edit-quantity']['widget']) or dpm($form['edit-quantity']['widget']).