必须先将用户附加到现有用户。

[HttpPost]
    public ActionResult Edit(ApplicationUser user, string password)
    {  

        var Existinguser = await UserManager.FindByNameAsync(user.userName);

        if(password.IsEmpty() == false)
        {
            Existinguser.PasswordHash = userManager.PasswordHasher.HashPassword(password);
        }
        Existinguser.Status = Enums.DataStatus.Updated;

        //...

        //Update the existing user here
        // ExistingUser.Address = user.Address;
        //..

        var result =userManager.Update(ExistingUser);  //Update existing user here

        return RedirectToAction("EmployeeList");
    }