gistfile1.sql

SELECT CASE
  WHEN ("json" -> "key_to_set") IS NULL THEN "json"
  ELSE COALESCE(
    (SELECT ('{' || string_agg(to_json("key") || ':' || "value", ',') || '}')
       FROM (SELECT *
               FROM json_each("json")
              WHERE "key" <> "key_to_set"
              UNION ALL
             SELECT "key_to_set", to_json("value_to_set")) AS "fields"),
    '{}'
  )::json
END