How do I use an UpdateView to update a Django Model?

It should be:

def get_object(self, queryset=None):
    obj = Portfolios.objects.get(id=self.kwargs['id'])
    return obj

Look at class based generic view dispatch explains that keyword arguments are assigned to self.kwargs.:

def dispatch(self, request, *args, **kwargs):
    # Try to dispatch to the right method; if a method doesn't exist,
    # defer to the error handler. Also defer to the error handler if the
    # request method isn't on the approved list.
    if request.method.lower() in self.http_method_names:
        handler = getattr(self, request.method.lower(), self.http_method_not_allowed)
    else:
        handler = self.http_method_not_allowed
    self.request = request
    self.args = args
    self.kwargs = kwargs
    return handler(request, *args, **kwargs)

id = self.request.GET.get('id',None) is what you needed when trying to access the GET query string.

However, your view can be simplified:

from django.conf.urls import *
from django.views.generic import UpdateView
from yourapp.models import Portfolios
from yourapp.forms import PortfoliosCreateForm

urlpatterns = patterns('',
    url('^portfolios/update/(?P<pk>[\w-]+)$', UpdateView.as_view(
        model=Portfolios,
        form_class=PortfoliosCreateForm,
        template_name='portfolios/create.html',
        success_url='/portfolios'
    ), name='portfolio_update'),
)