It depends on how you're trying to change it - you can really only adjust the postage costs, either by discounting or adding (for things like a different service, for example if the buyer has requested express and it wasn't offered in the original listing), and you can't discount the item price(s) at all.
On the item you've sold, there will be a dropdown menu where you can select Send Invoice, and you simply type in the adjusted postage amount (change the service being used if needed) then click send (or preview then send). If you click 'recalculate', the invoice will revert back to the original amount(s), so only use that if you want to start over.
