Surely the set d.ItemNumber = o.ItemNumber isn't needed as the "join" is on OrderNumber and ItemNumber:

MERGE OrderDetails AS d

   48 USING OrderInfo AS o

   49 ON (d.OrderNumber = o.OrderNumber AND d.ItemNumber = o.ItemNumber)

WHEN MATCHED THEN

   51     UPDATE SET

   52         d.ItemNumber = o.ItemNumber,

Regards

Leo