DECLARE @MyTable TABLE (Col1 INT,
                        Col2 INT)

--                          Col1   Col2
INSERT INTO @MyTable VALUES (1,     2)
INSERT INTO @MyTable VALUES (3,     4)

UPDATE @MyTable SET Col1 = Col2, Col2 = Col1

SELECT Col1, Col2 FROM @MyTable

DECLARE @MyTable TABLE (Col1 INT,
                        Col2 INT
,Col3 INT)
DECLARE @i INT
--                          Col1   Col2
INSERT INTO @MyTable(Col1,Col2) VALUES (1,     2)
INSERT INTO @MyTable(Col1,Col2) VALUES (3,     4)
SET @i = 0
UPDATE @MyTable SET 
  @i = @i + Col1
, Col1 = Col2
, Col2 = Col1
, Col3 = @i

SELECT Col1, Col2, Col3 FROM @MyTable