ELK migration from 5.4 to 6.6

Hi All,

I'm planning to upgrade my ELK stack in production from 5.4. to 6.6. I need your help for best practice and action items to achieve this goal.
Below are the details -

ES (5.4.3) -
20 nodes cluster,
Data size = 16 TB approx

Logstash (2.2.4)
Kibana (5.4.3)
Logstash-forwarder (0.4.0)

I'll upgrade to 5.6 first then to 6. Also I would use filebeat in place of logstash-forwarder.
Could you please help to share the steps what I need to do first?
Thanks in Advance.

Was the upgrade documentation insufficient for your needs?