In order to change a customer's reserved seat, you need to go to the order page (see instructions here).
On the view order page, select "edit / add" below the ticket table.
The floor plan will appear so that you can deselect the original seat and choose a different one. On the drop down menu you can choose a discount, as long as one has been set up in the pricelist.
When you make the changes you want, click "next" to move to the order page (this way your changes are saved).
Do not edit reservation orders made online before contacting the customer.