1) Upgrade Cluster Manager node
2) Enable the maintenance mode on the Cluster Manager node
2) Upgrade all search head nodes
3) Upgrade all search peer (indexers) nodes
4) Disable maintenance mode on the Cluster Manager node
5) Upgrade the Deployer server
6) Upgrade the Deployment Server
7) Upgrade the Heavy Forwarders and UFs
Back up $SPLUNK_HOME/etc before upgrading Splunk on any node.
Also, Splunk strongly recommends completing the entire upgrade process quickly, to avoid any possibility of incompatibilities between node types running different versions.