"mappings": {
  "mytype": {
    "properties": {
      "Myfield1": {
        "type": "nested",
        "properties": {
          "field1": {
            "type": "string"
          },
          "field2": {
            "type": "long"
          }
        }
      },
      "Title": {
        "type": "string"
      }
    }
  }
}

{
  "query": {
    "match": {
      "Title": "elasticsearch"
    }
  },
  "script": "ctx._source.Myfield1 = [{'nestfield1':'foo blabla...','nestfield2':100},{'nestfield1':'abc...','nestfield2':200}]"
}

"query" : {
    "match_all" : {}
},
"script" : "ctx._source.Myfield1 = Myfield1;",
"params": {
  "Myfield1": {
    "nestfield1": "foo blabla..."
  }
}

"query" : {
    "match_all" : {}
},
"script" : "ctx._source.Myfield1 = Myfield1; ctx._source.Myfield1.nestfield1 = ctx._source.Myfield1Nestfield1;  ctx._source.Myfield1.nestfield2 = ctx._source.Myfield1Nestfield2;",
"params": {
  "Myfield1": {
    "nestfield1": "init_data"
  }
}

POST indexname/type/_update_by_query
{
  "query": {
    "match": {
      "Title": "elasticsearch"
    }
  },
  "script": {
    "source": "ctx._source.Myfield1= params.mifieldAsParam",
    "params": {
      "mifieldAsParam": [
        {
          "nestfield1": "foo blabla...",
          "nestfield2": 100
        },
        {
          "nestfield1": "abc...",
          "nestfield2": 200
        }
      ]
    },
    "lang": "painless"
  }
}