Updating source with aggregation values

[uyth (Uy)]

Using Elasticsearch v. 6.2.4, I want to update a field in my document based on multiple aggregations done on a nested object. Something like this:

POST <my_index>/_doc/<doc_id>/_update
{
  "script": {
    "lang": "painless", 
    "source": """
      ctx._source.field_to_update = my_aggregation_value;
                  """,
    "params": {
      "my_aggregation_value":  <something>
    }
  },
    "aggs": {
    "<nested_object>": {
      "nested": {
        "path": "<nested_object>"
      },
      "aggs": {
        "<time_range>": {
          "filter": {
            "range": {
              "history.request_time": {
                "gte": "now-7d/d",
                "lte": "now"
              }
            }
          },
          "aggs": {
            "bucket_by_day": {
              "date_histogram": {
                "field": "<nested_object>.<time_event>",
                "interval": "day"
              },
              "aggs": {
                "my_aggregation": {
                  "aggregation_type": {...}
                }
              }
            }
          }
        }
      }
    }
  }
}

So I have three problems:

1. How do I update my document fields with aggregation values? Are there any way to set variables in params to aggregation values.
2. How can I update the root with aggregations in nested objects?
3. How can I select the last aggregation of the buckets?