Hi,
In HikaMarket or in HikaShop, the order management works the same.
When you edit an existing order, you process a manual operation. HikaShop or HikaMarket won't reprocess the discount price so if you want to change it, you need to edit the discount value.
The discount can be applied on specific products, on specific categories, on the shipping (or not) ; so it's not possible to recalculate the discount price without having the full configuration of the discount (which could be changed since the order has been made).
That's why, you need to edit it manually ; because you edit the order content manually.
Regards,