gistfile1.ps1

function InstallService ($p,$s,$n,$e,$v) {
    $stop = "$p\$s stop --environment=$e"
    $uninstall = "$p\$s uninstall --environment=$e"
    $install = "$p\$s install --environment=$e"
    $start = "$p\$s start --environment=$e"

  $serviceName = "$n$" + $e

	$service = Get-WmiObject -Class Win32_Service -Filter "Name='$serviceName'"

	if ($service) {
		stop-service $serviceName
		
		$servicePath = "HKLM:\System\CurrentControlSet\services\$n`$$e\"
		$description = "$n $e / $v"
		$path = "`"$p\$s`" -instance:$e -displayname `"$n (Instance: $e)`" -servicename:$n"

		Set-ItemProperty -path $servicePath -name Description -value "$description"
		Set-ItemProperty -path $servicePath -name ImagePath -value "$path"

		start-service $serviceName
	}
	else {
		invoke-expression $install
		invoke-expression $start
	}
}

gistfile2.ps1

. .\InstallService.ps1

$serviceInstallerName = 'YourApp.exe'
$serviceName = 'YourApp'
InstallService $OctopusOriginalPackageDirectoryPath $serviceInstallerName $serviceName $OctopusEnvironmentName $OctopusPackageVersion