You can’t remove or update vmhosts if they are in Cluster on SCVMM 2008 R2. For that reason, you have to update hosts with Powershell. If you are getting “Access Denied” message from your hosts, you can reassociate them with Powershell:

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$Credential = Get-Credential
Get-VMMManagedComputer | where {$_.State -eq "AccessDenied"} | Reassociate-VMMManagedComputer -Credential $Credential

After that, just update vmhost using SCVMM. Host will be ready.