How do you swap unique indexed column values in database?

CREATE TABLE testing
(
    cola int NOT NULL,
    colb CHAR(1) NOT NULL
);

CREATE UNIQUE INDEX UIX_testing_a ON testing(colb);

INSERT INTO testing VALUES (1, "b");
INSERT INTO testing VALUES (2, "a");

SELECT * FROM testing;

UPDATE testing
SET colb = CASE cola WHEN 1 THEN "a"
                WHEN 2 THEN "b"
                END
WHERE cola IN (1,2);

SELECT * FROM testing;

cola    colb
------------
1       b
2       a

cola    colb
------------
1       a
2       b

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