How to write Unit testing in c# Mvc? Method have sessions

In controller I have a method that name Login. In that method we use session. So how to write unit testing?

[HttpPost]
        public ActionResult Login(LoginModel lVM)
        {
            LoginViewModel model = UM.Authentication(lVM);
            if (model.UserId > 0)
            {


                Session["LoginMessage"] =  model.UserName;
                Session["UserID"] = model.UserId;

                IDataHelper ex = new DataHelper();

                return RedirectToAction("HomePage", "Newcontroller");
            }
            else
            {
                Session["LoginMessage"] = "Invalid UserName and Password";
                return RedirectToAction("Login", "Login");
            }
        }