I have completed a few Skype for Business installs lately and have had to revisit to apply the Skype for Business Cumulative Update and wanted to put the steps taken here for reference.
1 Check the topology replication status to ensure that replication is working and all is in sync. This is done by either checking the Skype for Business Control Panel or running the ‘Get-CsManagementStoreReplicationStatus’ from PowerShell and confirming.
2 Then check the list of the upgrade domains for the Enterprise Edition pool that is associated with a front-end server. To do this, run the ‘Get-CsPoolUpgradeReadinessState’ cmdlet on a front-end server.
This will confirm what is ready.
Should a front-end server be offline a screenshot similar to this is shown: