Change Add To Cart To Buy Now drupal Commerce
If you want to alter the text for a particular form you can alter it using hook_form_alter 

Get the ID of the form for the product display where you are trying to change the text &amp; do something like this:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id){//replace your module name with MYMODULE
  if(strpos($form_id, 'commerce-cart-add-to-cart-form-65') !== false){//my form ID is "commerce-cart-add-to-cart-form-65", replace it with your form ID.
    $form['submit']['#value'] = t('Buy Now');//Put the text which you want to make appear instead of "Add to cart".
  }
}


You could also use String Overrides Module to achieve the same.