How to change the 'Quantity' label in commerce-order-item-add-to-cart-form
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']).