where can i change characteristic_id in value

Hi,

That code is generated on the line:
$product_code = $element->product_code.'_'.implode('_',$key);

of the file administrator/components/com_hikashop/classes/product.php

To change and use the names instead of the ids stored in the $key array, you will have to load the name for each id from the hikashop_characteristic with a MySQL query.