I would write a stored procedure for this: check if the row exists, and if so UPDATE it. If it doesn't, do an INSERT instead:
CREATE PROCEDURE spInsertOrUpdate
@ItemCode INT,
@ItemName NVARCHAR
AS
BEGIN
UPDATE MyTable SET ItemCode = ItemCode + @ItemCode WHERE ItemName = @ItemName
IF @@ROWCOUNT=0
INSERT INTO MyTable (ItemCode, ItemName) VALUES (@ItemCode, @ItemName)
END