You can find upgrade documentation for the Elastic Stack here.
When upgrading the Elastic Stack you want to upgrade your components in the following order:
- Elasticsearch
- Kibana
- Logstash
- Beats
The previous version of Beats is designed to work with the next version of Elasticsearch, i.e. "Beats and Logstash 6.7 are compatible with Elasticsearch 7.0.1 to give you flexibility in scheduling the upgrade." but not the other way around.
Rolling upgrades are supported between minor versions: "Elasticsearch supports rolling upgrades between minor versions, from Elasticsearch 5.6 to 6.7, and from 6.7 to 7.0.1."
If you'd like to remove your Filebeat indices, you can follow these docs: https://www.elastic.co/guide/en/elasticsearch/reference/6.7/indices-delete-index.html, you'll want to use a wildcard expression of filebeat-*.