Hi,
Try this code..
CREATE TABLE #IREMS (ItemID INT, City VARCHAR(20))
INSERT INTO #IREMS (ItemID , City )
SELECT 11, 'a' UNION ALL
SELECT 22, 'b' UNION ALL
SELECT 33, 'a' UNION ALL
SELECT 44, 'hh' UNION ALL
SELECT 55, 'b' UNION ALL
SELECT 66, 'a'
SELECT * FROM #IREMS
UPDATE #IREMS SET City = CASE WHEN City='a' THEN 'b' WHEN City='b' THEN 'a' ELSE City END
SELECT * FROM #IREMS
DROP TABLE #IREMS