magento2 modify Elasticsearch data by adding fileds

I have magento2 elastic search data. I want to add some data into it. Here is result of current data of following elastic search query

GET /alpha-m2_en_catalog_product_*/_mapping

Now i want the result of this query should contain additional “fields” data as shown in below image

"fields":{
   "Keyword":{
      "type": "keyword"
      "ignore_above" : 256
   }
}

How is it possible in Magento2 to grammatically add this data in the result of

bin/magento indexer:reindex