请改用此：

{
  "script": "for (int i=0;i<ctx._source.b.size();i++) { item=ctx._source.b[i]; if (item['x'] == x_id) { ctx._source.b[i] = newobj} };",
  "params": {
    "x_id": "c1",
    "newobj": {
      "x": "c1",
      "y": 4222
    }
  },
  "lang": "groovy"
}