[Answered ]-How to select box value can be change in django?

You need to use little bit of AJAX/JQuery.

#models.py
class Country(models.Model):
     country = models.CharField(max_length=20)

class State(models.Model):
    state = models.Charfield(max_length=20)
    country = models.ForeignKey(state)

#views.py
def filter (request):
    try:
        kwargs = {smart_str('country'): request.GET['q']}
    except KeyError:
        raise Http404
    qs = State.objects.filter(**kwargs).values('pk', 'name')
    response = HttpResponse(
        content=dumps(list(qs)),
        mimetype='application/json'
    )
    return response



#urls.py
urlpatterns = patterns('',
         url(r'^locations/filter/state-by-country/$', 'filter',  name='state_filter')
        ...
)

Add this to your template

//JQuery chained select plugin
$(function() {
  $('#id_state').chainedSelect({
    parent: '#id_country',
    url: 'locations/filter/find-by-country',
    value: 'id',
    label: 'name'
  });
});