You can refund only a specific ticket (or tickets) of a sales order and not the entire order.
Go to the order page (you can see instructions here).
From the "Tickets" table, select the ticket (or tickets) you want to cancel and click "cancel" (below this table and not at the bottom of the page).
On the next pop-up you will be asked to confirm your choice.
If the order has been paid by credit card, in the next step you will be transfered to the cancellation order where you have to press '''complete the cancellation order'', at the bottom right of your screen. And finally, in the next pop-up window click on "yes" to confirm the action.
After the cancellation, it will be stated at the top left of your screen:
xxxxxxxxxx | cancellation of xxxxxxxxxx | completed
In the initial order page, the cancellation order will appear under the table "relative orders" and its status should be "completed".
Α re-issue order cannot be partially cancelled. You can cancel the entire order.
If the venue is numbered, you can see the seat number under the tickets table, in order to cancel the desired ticket.
If the event has unnumbered seats, you can check the ticket number to cancel the desired ticket.