Update data that has many to many relationship with Entity Framework

Question

I’m trying to update an object and the only thing that doesn’t update is his relationship from many to many the rest as name and surname update

    [HttpPost]
    public IActionResult AtualizarProfessor(int id, string nome, string sobrenome, List<int> listaDeIdDasTurmas)
    {

        Professor professor = new Professor(nome, sobrenome);
        professor.Id = id;
        if (listaDeIdDasTurmas == null || listaDeIdDasTurmas.Count() == 0)
            return View();

        foreach (var turmaId in listaDeIdDasTurmas)
        {
            Turma turma = _turmaRepository.ObterTurmaPeloId(turmaId);

            if (turma == null || turma.Id == 0)
                return View();

            var professorturma = new ProfessorTurma();

            professorturma.ProfessorId = professor.Id;
            professorturma.TurmaId = turma.Id;

            professor.Turmas.Add(professorturma);
        }

        if (!professor.ValidaProfessor())
        {
            ViewData["Message"] = "Envie os dados do professor de forma correta!";
            return View();
        }
        else
        {
            _professorRepository.Atualizar(professor);
        }

        return RedirectToAction("cadastrarprofessor", "Admin");
    }




    public void Atualizar(Professor professor)
    {
        using (var contexto = new MinosContext())
        {
            contexto.Professores.Update(professor);
            contexto.SaveChanges();
        }
    }