Thanks for the fast reply ajeh, I done a search and had two results which are as follows,

C:/xampp/xampp/htdocs/store/includes/languages/english/button_names.php

Line #28 : define('BUTTON_IMAGE_DOWNLOAD', 'button_download.gif');

C:/xampp/xampp/htdocs/store/includes/templates/template_default/templates/tpl_modules_downloads.php

Line #71 : <td class="centeredContent"><?php echo ($is_downloadable) ? '<a href="' . zen_href_link(FILENAME_DOWNLOAD, 'order=' . $last_order . '&id=' . $downloads->fields['orders_products_download_id']) . '">' . zen_image_button(BUTTON_IMAGE_DOWNLOAD, BUTTON_DOWNLOAD_ALT) . '</a>' : '&nbsp;'; ?></td>