Solution:
In this situation direct don’t try to edit in plugin files. instead of that try to search for hook.
Here is the code you can add it in function file. it will change text and place holder also
function md_custom_woocommerce_checkout_fields( $fields )
{
$fields['order']['order_comments']['placeholder'] = 'Special notes';
$fields['order']['order_comments']['label'] = 'Add your special note';
return $fields;
}
add_filter( 'woocommerce_checkout_fields', 'md_custom_woocommerce_checkout_fields' );