DECLARE @t TABLE(city VARCHAR(1))

INSERT INTO @t(city)

VALUES ('A'),( 'B'),( 'C'),( 'A'),( 'B'),( 'C'),( 'A');

-- do a select and not the results for comparison purposes

SELECT city FROM @t

UPDATE @t

SET city = CASE city

WHEN 'A' THEN 'B'

WHEN 'B' THEN 'A'

else city

END

-- select after update to compare with initial select

select * from @t