Update By Query | Alias

Elastic Stack Elasticsearch
4

That's why it's failing. Try on customers.handle.keyword instead:

POST /my-alias-index/_update_by_query
{
  "script": {
    "source": "ctx._source['customers.company'] = params.company",
    "lang": "painless",
    "params": {
      "company": "apple"
    }
  },
  "query": {
    "term": {
      "customers.handle.keyword": "search-user815"
    }
  }
}