Update data that has many-to-many relationship with Entity Framework

I'm trying to update an object and the only thing that does not update is its relationship from many to many the rest as first and last name 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();
        }
    }