How to avoid to not add new row in DB , When i Edit User's

Question

I have page , where i can Edit User's Email and Password not problem ,but there is  problem, when its send Data to Database , it will add new row everytime , when i just trying to edit User's Email, so how can i avoid that to not add new row in database , just send data in same row with out adding new row.

 

The code i have in my Action:

public ActionResult UserEdit(string Email)
{
    return View(new user {Email = Email });
}

[HttpPost]
[ValidateAntiForgeryToken]

public ActionResult UserEdit(user User)
{
    var db = new DataContext();
    var u = db.PX.Where(t => t.Email_ID == User.Email).FirstOrDefault();
    if (u == null)
        db.PX.Add(new A_S_PX { Email_ID = User.Email, PS = User.Password });
    else {
    u.PS = User.Password;
    u.Email_ID = User.Email;

    }
    db.SaveChanges();
    return View(new user { Email = User.Email });


}