How to substitute all empty/blank values by "Others" in Elasticsearch?

When I create a pie chart, I want to substitute all empty values of a field myfield (i.e. equal to "") by "Others". How can I do it in Kibana?

If it's impossible to do in Kibana, then how can I do it using Elasticsearch.

UPDATE:

I executed this query and it didn't give me any error:

GET myindex/entry/_update_by_query
{
  "query":{
    "term": {
      "myfield.keyword": {
        "value": ""
      }
    }
  },
  "script":{
    "inline": "ctx._source.myfield = 'Other'",
    "lang": "painless"
  }
}

I get this output:

{
  "took": 5,
  "timed_out": false,
  "total": 0,
  "updated": 0,
  "deleted": 0,
  "batches": 0,
  "version_conflicts": 0,
  "noops": 0,
  "retries": {
    "bulk": 0,
    "search": 0
  },
  "throttled_millis": 0,
  "requests_per_second": -1,
  "throttled_until_millis": 0,
  "failures": []
}

But when I check the values of myfield, I get the value "" again, instead of Other.

GET myindex/_search?
{
 "size":0,
   "aggs": {
    "months": {
     "terms" : {
      "field": "myfield"
     }
    }
   }
}

This is my index mapping:

PUT /myindex
{
    "mappings": {
      "entry": {
      "_all": {
        "enabled": false
      },
        "properties": {
          "Id": {
            "type":"keyword"
          },
          "Country": {
            "type":"keyword"
          },
          "myfield": {
            "type":"keyword"
          },
          "Year": {
            "type":"integer"
          },
          "Counter": {
            "type":"integer"
          }
        }
      }
    }
}