how PostgreSQL update the column type json with json array

Here are two helper functions, to achieve your goal (requires PostgreSQL 9.3+):

This one can be used like UPDATEs (only updates an index, if it's already exists):

CREATE OR REPLACE FUNCTION "json_array_update_index"(
  "json"            json,
  "index_to_update" INTEGER,
  "value_to_update" anyelement
)
  RETURNS json
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$
SELECT concat('[', string_agg("element"::text, ','), ']')::json
  FROM (SELECT CASE row_number() OVER () - 1
                 WHEN "index_to_update" THEN to_json("value_to_update")
                 ELSE "element"
               END "element"
          FROM json_array_elements("json") AS "element") AS "elements"
$function$;

This one can be used, like an UPSERT (updates an index, if it exists, or creates, if not -- using some default value to fill up unused indexes):

CREATE OR REPLACE FUNCTION "json_array_set_index"(
  "json"            json,
  "index_to_set"    INTEGER,
  "value_to_set"    anyelement,
  "default_to_fill" json        DEFAULT 'null'
)
  RETURNS json
  LANGUAGE sql
  IMMUTABLE
  STRICT
AS $function$
SELECT concat('[', string_agg((CASE "index"
                                 WHEN "index_to_set" THEN to_json("value_to_set")
                                 ELSE COALESCE("json" -> "index", "default_to_fill")
                               END)::text, ','), ']')::json
  FROM generate_series(0, GREATEST("index_to_set", json_array_length("json") - 1)) AS "index"
$function$;

With these, you can UPDATE any json data, like:

UPDATE plan_base
SET    atts = json_array_update_index(atts, 1, '{"planId":"71"}'::json)
WHERE  id = 46;

Important! Json arrays are indexed from 0 (unlike other PostgreSQL arrays). My functions respect this kind of indexing.

SQLFiddle

More about updating a JSON object:

• How do I modify fields inside the new PostgreSQL JSON datatype?

Update: functions are now compacted.