UPDATE t
SET cancelled = CASE WHEN Seq=1 THEN 1 ELSE 0 END
FROM (SELECT ROW_NUMBER() OVER (PARTITION BY refnumber ORDER BY dateadded DESC) AS seq,cancelled
FROM table
)t

DECLARE @example TABLE(refnumber VARCHAR(20), dateadded date, cancelled BIT)
INSERT INTO @example
SELECT '48394A0021', '2002-01-01', 0 union all  -- Single refnumber
SELECT '54739A0040', '2002-01-01', 0 union all  -- Single refnumber
SELECT '54739A0150', '2002-01-01', 0 union all
SELECT '54739A0150', '2004-01-01', 1            -- marked as 1 b'coz latest record for refnumber '54739A0150'
UPDATE @example SET
cancelled = 1 
FROM @example e 
JOIN (SELECT refnumber, MAX(dateadded) RecentDate
		FROM @example
		GROUP BY refnumber
	 ) t
ON e.refnumber= t.refnumber AND e.dateadded= t.RecentDate

UPDATE @example SET
cancelled = CASE WHEN t.refnumber IS NOT NULL THEN  1 ELSE 0 END
FROM @example e 
LEFT JOIN (SELECT refnumber, MAX(dateadded) RecentDate
		FROM @example
		GROUP BY refnumber
	 ) t
ON e.refnumber= t.refnumber AND e.dateadded= t.RecentDate