Plugin code to update a field

Hi 

I am new to Plugin in D365, and stuck on updating a field when two field is concated and displayed to 3 field where I have updated a lookup field and now to be updated
following is my concated code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.ServiceModel;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Query;

namespace TestTest189
{
public class Class1 : IPlugin
{
public void Execute(IServiceProvider serviceProvider)
{
// Obtain the tracing service
ITracingService tracingService =
(ITracingService)serviceProvider.GetService(typeof(ITracingService));

// Obtain the execution context from the service provider.
IPluginExecutionContext context = (IPluginExecutionContext)
serviceProvider.GetService(typeof(IPluginExecutionContext));

// The InputParameters collection contains all the data passed in the message request.
if (context.InputParameters.Contains("Target") &&
context.InputParameters["Target"] is Entity)
{
// Obtain the target entity from the input parameters.
Entity entity = (Entity)context.InputParameters["Target"];

// Obtain the organization service reference which you will need for
// web service calls.
IOrganizationServiceFactory serviceFactory =
(IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

try
{
if (entity.LogicalName == "salesorder")
{

try
{
Entity test = new Entity("salesorder");

string order_no = entity.GetAttributeValue<String>("ordernumber");

EntityReference Customer = (EntityReference)entity.Attributes["customerid"];

var RetrviName = service.Retrieve(Customer.LogicalName, Customer.Id, new ColumnSet("name"));
//Getting Customer Name
var AccountName = RetrviName["name"].ToString();

var concat = order_no + " " + AccountName;
entity["name"] = concat;
service.Update(entity);

}
catch (FaultException<OrganizationServiceFault> ex)
{
throw new InvalidPluginExecutionException("invalidException", ex);
}
}
}

catch (FaultException<OrganizationServiceFault> ex)
{
throw new InvalidPluginExecutionException("An error occurred in FollowUpPlugin.", ex);
}

catch (Exception ex)
{
tracingService.Trace("FollowUpPlugin: {0}", ex.ToString());
throw;
}
}
}
}
}