Tag - DAG

Please ensure you have a backup of your Exchange server before doing this

Download and extract the cumulative update

Open CMD prompt (make sure you run as administrator) and run these commands from the extracted directory

Setup.exe /prepareSchema /IAcceptExchangeServerLicenseTerms
Setup.exe /prepareAD /IAcceptExchangeServerLicenseTerms
Setup.exe /prepareDomain /IAcceptExchangeServerLicenseTerms

Put 1st DAG server into maintenance mode (use exchange powershell)

$server = $ENV:ComputerName
Set-ServerComponentState $server -Component HubTransport -State Draining -Requester Maintenance
Redirect-Message -Server $server -Target nameof2ndserver
Suspend-ClusterNode $server
Set-MailboxServer $server -DatabaseCopyActivationDisabledAndMoveNow $True
Set-MailboxServer $server -DatabaseCopyAutoActivationPolicy Blocked
Set-ServerComponentState $server -Component ServerWideOffline -State Inactive -Requester Maintenance

Close exchange powershell and open CMD prompt (as administrator user)

Setup.exe /mode:upgrade /IAcceptExchangeServerLicenseTerms

When finished, open exchange powershell and take server out of maintenance mode

$server = $ENV:ComputerName
Set-ServerComponentState $server -Component ServerWideOffline -State Active -Requester Maintenance
Resume-ClusterNode $server
Set-MailboxServer $server -DatabaseCopyActivationDisabledAndMoveNow $False
Set-MailboxServer $server -DatabaseCopyAutoActivationPolicy Unrestricted
Set-ServerComponentState $server -Component HubTransport -State Active -Requester Maintenance
Restart-Service MSExchangeTransport
Restart-Service MSExchangeFrontEndTransport

Then run the same process on the other Exchange DAG server