Update certain array elements of a json array in PostgreSQL 9.4

You can do this from PostgreSQL 9.5 with jsonb_set:

INSERT INTO test(data) VALUES('[1,2,"a",4,"8",6]');
UPDATE test SET data = jsonb_set(data, '{2}','"b"', false) WHERE id = 1

Try it out with a simple select:

SELECT jsonb_set('[1,2,"a",4,"8",6]', '{2}','"b"', false)
-- [1, 2, "b", 4, "8", 6]

And if you want to update two fields you can do:

SELECT jsonb_set(jsonb_set('[1,2,"a",4,"8",6]', '{0}','100', false), '{2}','"b"', false)
-- [100, 2, "b", 4, "8", 6]

For Postgres 9.5 or later use jsonb_set(). See later answer of adriaan.

---

You cannot manipulate selected elements of a json / jsonb type directly. Functionality for that is still missing in Postgres 9.4. You have to do 3 steps:

1. Unnest / decompose the JSON value.
2. Manipulate selected elements.
3. Aggregate / compose the value back again.

To replace the 3rd element of the json array (data->3) in the row with id = 1 with a given (new) value ('<new_value>'):

UPDATE test t
SET    data = t2.data
FROM  (
   SELECT id, array_to_json(
                 array_agg(CASE WHEN rn = 1 THEN '<new_value>' ELSE elem END))
              ) AS data
   FROM   test t2
        , json_array_elements_text(t2.data) WITH ORDINALITY x(elem, rn)         
   WHERE  id = 1
   GROUP  BY 1
   ) t2
WHERE  t.id = t2.id
AND    t.data <> t2.data; -- avoid empty updates

About json_array_elements_text():

• How to turn JSON array into Postgres array?

About WITH ORDINALITY:

• PostgreSQL unnest() with element number