Follow instruction of step 3 - Upgrade the peer node tier on that indexer node.
https://docs.splunk.com/Documentation/Splunk/7.3.1/Indexer/Upgradeacluster#Upgrade_each_tier_separately
So basically,
Run splunk enable maintenance-mode on the master. To confirm that the master is in maintenance mode, run splunk show maintenance-mode on the master. This step prevents unnecessary bucket fix-ups.
Stop Splunk on the peer nodes. When bringing down the peer, use the splunk stop command, not
splunk offline.
Upgrade the peer nodes, following the normal procedure for any Splunk Enterprise upgrade.
Start the peer nodes, if they
are not already running.
Run splunk disable maintenance-mode on the master.
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