小编徐巧民*_*徐巧民的帖子

{
  "_id": "xPBc4By8FemDwTPqH",
  "u": {
    "_id": "6PoZawHZcQz4Gwzcv",
    "username": "michael"
  },
  "friends": [
    {
      "u": {
        "_id": "eGqDjAjjtYADbuSnn",
        "username": "michael",
        "name": "michael"
      }
    },
    {
      "u": {
        "_id": "k4gKCGwYryXDMMHvs",
        "username": "joyce",
        "name": "joyce"
      }
    }
  ]
}

我想更新"friends.u.username"的名称:"michael"的名字是"你好",我需要怎么做.