更新如果已存在并插入数据

如何解决更新如果已存在并插入数据

我开发了一个测验系统，如果用户重复测验，我希望测验分数会更新。我不知道该怎么做，因为如果他们单击按钮，idscore不会保存任何值。

控制器

[HttpPost]
public ActionResult Insertscore([Bind(Include = "idscore,date,status,IdMaterial,score,username")] scoreQ s)
{
    if (Session["UserName"] != null)
    {
        var user = Session["UserName"].ToString();
        bool isValid = db.scoreQs.Any(x => x.username == user &&  x.IdMaterial== s.IdMaterial );

        if (isValid)
        {
            var quizz = db.scoreQs.Where(x => x.MaterialLearning.id == s.IdMaterial && x.username == user).Select(a => a.idscore );

            // scoreQ existing = db.scoreQs.Find(quizz);

            if (ModelState.IsValid)
                db.Entry(s).State = EntityState.Modified;

            db.SaveChanges();
            return Json("true",JsonRequestBehavior.AllowGet);
        }
        else
        {
            s.username = user;
            db.scoreQs.Add(s);
            db.SaveChanges();
            return Json("true",JsonRequestBehavior.AllowGet);
        }
    }
    else
    {
        return RedirectToAction("Login","Account");
    }
}

.js

function restartQuiz() {
    start.style.display = "block";
    scoreBlock.style.display = "none";
    scoreMessage.style.display = "none";
    quizAgain.style.display = "none";
    score = 0;
    questionIndex = 0;
}