It depends on how the purchase was made. If they purchased two items singularly (i.e. confirming a quantity of one in two separate purchases), you can just send a new invoice for one by unticking one of the items then sending through a cancellation request for the other (it's best to explain to the buyer exactly what's happening as they will need to agree to the cancellation in order for the FVF to be refunded).
If, on the other hand, they only went through the buy-now process once and purchased a quantity two, then you can't actually change the invoice to be for only one item. The only (proper) option then would be to cancel the first purchase and have them buy again, this time making sure they only purchase one.
