Updates an existing index in an OpenSearch Serverless collection. This operation allows you to modify the index schema, including adding new fields or changing field mappings. You can also enable automatic semantic enrichment ingestion and search. For more information, see <a href="https://docs.aws.amazon.com/opensearch-service/latest/developerguide/serverless-manage.html#serverless-semantic-enrichment">About automatic semantic enrichment</a>.