$primaryMS = Get-ManagementServer | where {your filter here}
$failoverMS = Get-ManagementServer | where {your filter here}
$gatewayMS = Get-ManagementServer | where {your filter here}
Set-ManagementServer -GatewayManagementServer: $gatewayMS -PrimaryManagementServer: $primaryMS -FailoverServer: $failoverMS