Hi,
Just had the same issue, you need to update the template for filebeat & es6.
You have the new mapping file in /etc/filebeat/filebeat.template-es6.json
I deleted the previous one and load the new one but there may be a better option if you need to keep your data.
curl -XDELETE http://localhost:9200/_template/filebeat
curl -XPUT -H 'Content-Type: application/json' http://localhost:9200/_template/filebeat -d@filebeat.template-es6x.json
See https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-template.html
Maybe there is a smarter way with some alias ?