Change Add to Cart button label in Drupal Commerce for specific product.

template.php

<?php
/**
* Implements hook_form_alter
*
*/

function arma_form_alter(&$form, $form_state, $form_id) {
  if ($form_id === 'commerce_cart_add_to_cart_form_2' || $form_id === 'commerce_cart_add_to_cart_form_1') { 
      $form['submit']['#attributes']['title'] = $form['submit']['#attributes']['value'] = t('Buy Corporate Membership');
    }
  if ($form_id === 'commerce_cart_add_to_cart_form_4' || $form_id === 'commerce_cart_add_to_cart_form_3') { 
      $form['submit']['#attributes']['title'] = $form['submit']['#attributes']['value'] = t('Buy Individual Membership');
    }
}