Apache Beam Elastic IO模块可以使用Elasticsearch提供的update API来更新现有文档。以下是一个使用Beam Elastic IO模块更新文档的Python示例代码：

import apache_beam as beam
from apache_beam.io.elasticsearch import WriteToElasticsearch
from elasticsearch.helpers import bulk_index, bulk_update

# Define a function to update the document
def update_doc(doc):
    doc_id = doc["_id"]
    # Set the update operation and new field values
    update = {"doc": {"new_field": "new_value"}}
    # Call the bulk_update function to update the document
    bulk_update(es, (update, doc_id))
    return doc

# Set up the Beam pipeline to read from Elasticsearch and update documents
options = {'es.resource': 'myindex/mytype', 'es.nodes': 'localhost', 'es.port': '9200'}
with beam.Pipeline() as p:
    rows = p | 'ReadFromES' >> ReadFromElasticsearch(**options)
    updated_rows = rows | 'UpdateDoc' >> beam.Map(update_doc)
    updated_rows | 'WriteToES' >> WriteToElasticsearch(**options)

此代码将使用Elasticsearch Python客户端的bulk_update函数来更新文档，将新字段添加到现有文档中。在Elasticsearch中，更新文档的操作是将一个新版本的文档完全替换原始文档。如果您需要部分更新文档，请使用部分更新的API。