Postgres 11

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE tab ( jsdata JSONB )
 INSERT INTO tab VALUES 
 ('{"john": 34, "alex": 56, "daniel": 90}')
1 rows affected

UPDATE tab
   SET jsdata = jsdata || '{"harry":78}'::JSONB || '{"john":39}'::JSONB 
1 rows affected

SELECT *
  FROM tab


jsdata


{"alex": 56, "john": 39, "harry": 78, "daniel": 90}