I have done a clean install of Suricata and the ELK stack version 6.2.1 ... "Your Kibana Index is out of date, reset it or use the X-Pack upgrade assistant."
I'm not sure how this is possible, out of date indexes must be created with a 5.x version of the Elastic stack. Are you importing data from an older installation?
One suggestion was to install X-Pack 5.6 and use it to upgrade the indices but I don't understand the relationship between X-Pack and the ELK stack. To use X-Pack 5.6 would I need to install the ELK 5.6 stack also?
yes, but that shouldn't be necessary if you're doing a clean install.
I understand that if the Index is reset, Kibana will rebuild it on the next startup. Is there a simple way to reset the Index?
Yep, deleting it from Elasticsearch with curl should do the trick:
curl -XDELETE -u elastic -p http://localhost:9200/.kibana