Hi,
I don't see a nice way of doing this via the interface provided in the backend.
The best will be to run a MySQL query like below directly from your PHPMyAdmin:
SELECT c.category_name, sum((op.order_product_quantity) AS sales FROM #__hikashop_category as c LEFT JOIN #__hikashop_product AS p ON c.category_id =p.product_manufacturer_id LEFT JOIN #__hikashop_product AS v ON p.product_id = v.product_parent_id LEFT JOIN #__hikashop_order_product AS op ON op.product_id=p.product_id OR op.product_id=v.product_id LEFt JOIN #__hikashop_order AS o ON op.order_id = o.order_id WHERE o.order_status in ('confirmed', 'shipped');
you need to replace the #__ with the table prefix of your tables.
As you can see, that's a complex MySQL query.