Upgrading ES 5.2 to ES 5.6.13

Hello, all
My staging stage is ES 5.2 right now. I wanna upgrade ES to be 5.6.13.
Is this procedure very safe ? https://www.elastic.co/guide/en/elasticsearch/reference/5.6/rolling-upgrades.html
Example, i have 5 servers in my ES cluster. Which one should be upgraded first, node data, node ingest or node master ?
Even though this is staging stage, i wanna try the best one, so in the next case or production stage, i can implement it by myself. Thank you for your time and help.

Solved, i need to upgrade it 1 by 1.
This thread is very clear https://www.elastic.co/guide/en/elasticsearch/reference/5.6/rolling-upgrades.html

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