Разобраться помог 
Сергей Горностаев, решение выглядит так:
 
class UserSettingsViews(LoginRequiredMixin, View):
    """Отображение и наверное изменение пользовательских настроек оповещения """
    def post(self, request, pk):
        settings_exist = get_object_or_404(ObjectsOfSurveillance, user_id=pk)
        if request.method == 'POST':
            
            settings_form = ChangeSettingsForm(request.POST, instance=settings_exist)
            if settings_form.is_valid():
                settings_form.save()
                messages.success(request, 'Настройки успешно обновлены!')
        return render(request, 'main/user_settings_pg.html', {'settings_form': settings_form})
    def get(self, request):
        settings_exist = get_object_or_404(ObjectsOfSurveillance, user_id=request.user.id)
        settings_form = ChangeSettingsForm(instance=settings_exist)
        return render(request, 'main/user_settings_pg.html', {'settings_form': settings_form})