-- 1. SubQuery
UPDATE t1
   SET c = (SELECT c FROM t2 WHERE a = t1.a AND b = t1.b)
 WHERE EXISTS (SELECT 1 FROM t2 WHERE a = t1.a AND b = t1.b)
-- WHERE c IN (SELECT c FROM t2)
;

-- 2. Updatable Join View
UPDATE 
(
SELECT t1.c old_c
     , t2.c new_c
  FROM t2
 WHERE t1.a = t2.a
   AND t1.b = t2.b
)
SET old_c = new_c
;

-- 3. Merge
MERGE t1
USING t2
   ON (t1.a = t2.a AND t1.b = t2.b)
 WHEN MATCHED THEN
      UPDATE SET c = t2.c
;