How to update Column related to Row Number Is there any easy way to update Column by Row number not a PK 
ex: 

UPDATE contact m
SET    ContactNumber = sub.rn + 500
FROM  (SELECT Id, row_number() OVER (ORDER BY Id) AS rn FROM contact) sub
WHERE  m.Id = sub.Id;
UPDATE contact
SET ContactNumber = (@rownum := 1 + @rownum) + 500
WHERE 0 = (@rownum:=0)
-- for a given client, for example, Client_Id=123, use
-- WHERE Client_Id = 123 + (@rownum:=0)
ORDER BY id;