We have this one documented in our Online Help:
There are some points highlighted but thought worth mentioning again:
Important: You should not upgrade a worker node before upgrading the primary node. Start the upgrade on the primary node, and if prompted by the setup process, move to the worker nodes and manually upgrade them before returning to the primary node to complete the upgrade. If you upgrade a worker node before upgrading the primary node, you will need to completely uninstall Tableau Server, reinstall, and restore from your backup.
With most upgrades, the worker nodes will be automatically updated.
Note: When worker nodes are upgraded automatically, the Windows registry on the worker nodes will not reflect the upgrade, so the old version will still show in the program list of Control Panel. This is expected.
Thank you Mark.