Kibana Logstash ElasticSearch | Unindexed Fields

You've updated the Kibana field list?

1. Kibana.
2. Settings.
3. Reload field list.

Newer version:

1. Kibana.
2. Management.
3. Refresh icon on the top right.

As of 6.4.0:

The warning description puts it very simply:

Management > Index Patterns > Select your Index > Press the refresh button in the top right corner.