A user accidently hit 2 for quantity when 1 was intended. The CC payment has already gone through.

I underestand that the payment refund needs to be made at the gateway portal.

But I'm attempting to update the order record to set the quantity to 1 and then also adjust the money amount. It looks like adjusting the money is not so hard, but quantity wants to remain read-only in all the edit screens.

How should I instruct my client to proceed?