edit_post.html

{% extends 'base.html' %}

{% block title %}Edit post{% endblock %}

{% block breadcrumb %}
  <li class="breadcrumb-item"><a href="{% url 'home' %}">Boards</a></li>
  <li class="breadcrumb-item"><a href="{% url 'board_topics' post.topic.board.pk %}">{{ post.topic.board.name }}</a></li>
  <li class="breadcrumb-item"><a href="{% url 'topic_posts' post.topic.board.pk post.topic.pk %}">{{ post.topic.subject }}</a></li>
  <li class="breadcrumb-item active">Edit post</li>
{% endblock %}

{% block content %}
  <form method="post" class="mb-4" novalidate>
    {% csrf_token %}
    {% include 'includes/form.html' %}
    <button type="submit" class="btn btn-success">Save changes</button>
    <a href="{% url 'topic_posts' post.topic.board.pk post.topic.pk %}" class="btn btn-outline-secondary" role="button">Cancel</a>
  </form>
{% endblock %}

urls.py

from django.conf.urls import url
from django.contrib import admin
from django.contrib.auth import views as auth_views

from accounts import views as accounts_views
from boards import views

urlpatterns = [
    url(r'^$', views.home, name='home'),
    url(r'^signup/$', accounts_views.signup, name='signup'),
    url(r'^login/$', auth_views.LoginView.as_view(template_name='login.html'), name='login'),
    url(r'^logout/$', auth_views.LogoutView.as_view(), name='logout'),

    url(r'^reset/$',
        auth_views.PasswordResetView.as_view(
            template_name='password_reset.html',
            email_template_name='password_reset_email.html',
            subject_template_name='password_reset_subject.txt'
        ),
        name='password_reset'),
    url(r'^reset/done/$',
        auth_views.PasswordResetDoneView.as_view(template_name='password_reset_done.html'),
        name='password_reset_done'),
    url(r'^reset/(?P<uidb64>[0-9A-Za-z_\-]+)/(?P<token>[0-9A-Za-z]{1,13}-[0-9A-Za-z]{1,20})/$',
        auth_views.PasswordResetConfirmView.as_view(template_name='password_reset_confirm.html'),
        name='password_reset_confirm'),
    url(r'^reset/complete/$',
        auth_views.PasswordResetCompleteView.as_view(template_name='password_reset_complete.html'),
        name='password_reset_complete'),

    url(r'^settings/password/$', auth_views.PasswordChangeView.as_view(template_name='password_change.html'),
        name='password_change'),
    url(r'^settings/password/done/$', auth_views.PasswordChangeDoneView.as_view(template_name='password_change_done.html'),
        name='password_change_done'),

    url(r'^boards/(?P<pk>\d+)/$', views.board_topics, name='board_topics'),
    url(r'^boards/(?P<pk>\d+)/new/$', views.new_topic, name='new_topic'),
    url(r'^boards/(?P<pk>\d+)/topics/(?P<topic_pk>\d+)/$', views.topic_posts, name='topic_posts'),
    url(r'^boards/(?P<pk>\d+)/topics/(?P<topic_pk>\d+)/reply/$', views.reply_topic, name='reply_topic'),
    url(r'^boards/(?P<pk>\d+)/topics/(?P<topic_pk>\d+)/posts/(?P<post_pk>\d+)/edit/$',
        views.PostUpdateView.as_view(), name='edit_post'),
    url(r'^admin/', admin.site.urls),
]

views.py

from django.db.models import Count
from django.contrib.auth.decorators import login_required
from django.shortcuts import get_object_or_404, redirect, render
from django.views.generic import UpdateView
from django.utils import timezone

from .forms import NewTopicForm, PostForm
from .models import Board, Post, Topic


def home(request):
    boards = Board.objects.all()
    return render(request, 'home.html', {'boards': boards})


def board_topics(request, pk):
    board = get_object_or_404(Board, pk=pk)
    topics = board.topics.order_by('-last_updated').annotate(replies=Count('posts') - 1)
    return render(request, 'topics.html', {'board': board, 'topics': topics})


@login_required
def new_topic(request, pk):
    board = get_object_or_404(Board, pk=pk)
    if request.method == 'POST':
        form = NewTopicForm(request.POST)
        if form.is_valid():
            topic = form.save(commit=False)
            topic.board = board
            topic.starter = request.user
            topic.save()
            Post.objects.create(
                message=form.cleaned_data.get('message'),
                topic=topic,
                created_by=request.user
            )
            return redirect('topic_posts', pk=pk, topic_pk=topic.pk)
    else:
        form = NewTopicForm()
    return render(request, 'new_topic.html', {'board': board, 'form': form})


def topic_posts(request, pk, topic_pk):
    topic = get_object_or_404(Topic, board__pk=pk, pk=topic_pk)
    topic.views += 1
    topic.save()
    return render(request, 'topic_posts.html', {'topic': topic})


@login_required
def reply_topic(request, pk, topic_pk):
    topic = get_object_or_404(Topic, board__pk=pk, pk=topic_pk)
    if request.method == 'POST':
        form = PostForm(request.POST)
        if form.is_valid():
            post = form.save(commit=False)
            post.topic = topic
            post.created_by = request.user
            post.save()
            return redirect('topic_posts', pk=pk, topic_pk=topic_pk)
    else:
        form = PostForm()
    return render(request, 'reply_topic.html', {'topic': topic, 'form': form})


class PostUpdateView(UpdateView):
    model = Post
    fields = ('message', )
    template_name = 'edit_post.html'
    pk_url_kwarg = 'post_pk'
    context_object_name = 'post'

    def form_valid(self, form):
        post = form.save(commit=False)
        post.updated_by = self.request.user
        post.updated_at = timezone.now()
        post.save()
        return redirect('topic_posts', pk=post.topic.board.pk, topic_pk=post.topic.pk)

Update for path:
path('boards/int:pk/topics/int:topic_pk/posts/int:post_pk/edit/',
views.PostUpdateView.as_view(), name='edit_post'),