// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
     $fields['order']['order_comments']['placeholder'] = 'My new placeholder';
     $fields['order']['order_comments']['label'] = 'My new label';
     return $fields;
}