How to increase Code coverage for pre-defined Controller in c# MVC - asp.net-mvc

Question

I have to write unit test case and increase the code coverage for a pre written controller.
using System.Collections.Generic;
using Newtonsoft.Json;
using System;
using System.Web.Mvc;
using TW.Data.Business;
using TW.Data.Service;
using TW.Web.Models;
using TW.Common.Utility;
namespace TW.Web.Controllers
{
[Authorize]
public class BillinInfoSearchController : Controller
{
private IBillinInfoSearch _IRepository;
private IResponseMessage _ServiceResponse;
public BillinInfoSearchController(IBillinInfoSearch irepository, IResponseMessage serviceresponse)
{
_IRepository = irepository;
_ServiceResponse = serviceresponse;
}
public ActionResult ActionMethod()
{
BillinInfoSearchViewModel viewModel = new BillinInfoSearchViewModel();
viewModel.Success = false;
try
{
if (!Sitecore.Context.PageMode.IsExperienceEditor)
{
//Getting Custom Properties
var userBillAccountNumber = Sitecore.Context.User.Profile.GetCustomProperty(UserProfileDetails.BillAccountNumber);
var userObjectIdentifier = Sitecore.Context.User.Profile.GetCustomProperty(UserProfileDetails.ObjectIdentifier);
if (!string.IsNullOrEmpty(userBillAccountNumber) && !string.IsNullOrEmpty(userObjectIdentifier))
{
viewModel.ModelList = new List<BillinInfoSearchModel>();
_ServiceResponse = _IRepository.SubmitModelDetails(userBillAccountNumber, userObjectIdentifier);
if (_ServiceResponse.Success && !string.IsNullOrEmpty(_ServiceResponse.Content) && _ServiceResponse.Content.Contains("Top_GUID")
&& !Sitecore.Context.PageMode.IsExperienceEditorEditing)
{
List<BillinInfoSearchModel> objectList = JsonConvert.DeserializeObject<List<BillinInfoSearchModel>>(_ServiceResponse.Content);
if (objectList.Count > 0)
{
viewModel.Success = true;
viewModel.ModelList = objectList;
}
}
}
}
}
catch (Exception ex)
{
Sitecore.Diagnostics.Log.Error(ex.Message, this);
}
return View("~/Views/OAM/BillinInfoSearchView.cshtml", viewModel);
}
}
}
I wrote one Unit test case and code coverage comes to 24%. If i divide the controller code into small piece of code & methods and then call those in Unit test class, coverage increasing to 65%,but code is deployed in UAT and dont want to update the deployed code.
Is there we can increasing the code coverage without modifying the existing controller
using Microsoft.VisualStudio.TestTools.UnitTesting;
using NSubstitute;
using System.Web.Mvc;
using TW.Data.Business;
using TW.Data.Service;
using TW.Web.Controllers;
using TW.Common.Utility;
namespace TW.UnitTest.Controller
{
[TestClass]
public class BillinInfoSearchControllerTest
{
[TestMethod]
public void BillinInfoSearchControllerViewName()
{
IBillinInfoSearch _IRepository = Substitute.For<IBillinInfoSearch>();
IResponseMessage _ServiceResponse = Substitute.For<IResponseMessage>();
BillinInfoSearchController twLiveController = new BillinInfoSearchController(_IRepository, _ServiceResponse);
Sitecore.Context.User.Profile.SetCustomProperty(UserProfileDetails.BillAccountNumber, "102134");
Sitecore.Context.User.Profile.SetCustomProperty(UserProfileDetails.ObjectIdentifier, "ef84bc1c-825c-4d8c-9801-3c3fc511a40e");
ViewResult action = twLiveController.ActionMethod() as ViewResult;
Assert.AreEqual("~/Views/OAM/BillinInfoSearchView.cshtml", action.ViewName);
}
}
}