WITH SalesCTE(SalesYear, SetQty, SetValue, GetQty, GetValue)
AS
(  SELECT Cur.SalesYear,
    Cur.PrevQty, Cur.PrevValue, Prev.Qty, Prev.Value
  FROM dbo.SalesSummary AS Cur
    LEFT OUTER JOIN dbo.SalesSummary AS Prev
      ON Prev.SalesYear = Cur.SalesYear – 1  )
UPDATE SalesCTE
  SET SetQty = GetQty, SetValue = GetValue;