This would require some code changes as well. First update the docker-compose.yaml file to get the image for elasticsearch version 6.3.2

    # docker-compose.yaml
    services:
      elasticsearch:
        image: docker.elastic.co/elasticsearch/elasticsearch:6.3.2

Elasticsearch 6 has some breaking changes. The mapping type string is no longer allowed for indexing. It is split into text and keyword. keyword is used for aggregation, sorting etc while text supports full text searching. Change the existing indexes with type string to text and keyword based on the requirements.

The old indexes made by elasticsearch for string type will still be read if the indexes were made on elasticsearch 5.