首先修改app/controllers/posts_controller.rb，添加

  def edit
    @group = Group.find(params[:group_id])
    @post = @group.posts.find(params[:id])
  end

  def update
    @group = Group.find(params[:group_id])
    @post = @group.posts.find(params[:id])

    if @post.update(post_params)
      redirect_to account_posts_path, notice: "Update Success"
    else
      render :edit
    end
  end

  def destroy
    @group = Group.find(params[:group_id])
    @post = @group.posts.find(params[:id])
    @post.destroy
    flash[:alert] = "Post deleted"
    redirect_to account_posts_path
  end

新增一个编辑页面app/views/posts/edit.html.erb

<div class="col-md-4 col-md-offset-4">
  <h2>编辑文章内容</h2>
  <hr>
  <%= simple_form_for [@group, @post] do |f| %>
    <div class="form-group">
      <%= f.input :content, input_html: { class: "form-control"} %>
    </div>
    <div class="form-actions">
      <%= f.submit "Submit", disable_with: "Submiting...", class: "btn btn-primary" %>
    </div>
  <% end %>
</div>

完成编辑和删除文章功能