$output = Get-AzureRmVM | where-object {$_.LicenseType -NE "Windows_Server" -and $_.StorageProfile.osDisk.OsType -eq "Windows"} | select Name,ResourceGroupName

$rgname = $output.Name
foreach($item in $rgname)
{
    $vm = Get-AzureRmVM -Name $item -ResourceGroup $output.ResourceGroupName
    $vm.LicenseType = "Windows_Server"
    Update-AzureRmVM -ResourceGroupName $item -VM $vm
}