title: Update a particular field in the entire django model
body: <p>I have a model</p>
  
  <pre><code>class College(models.Model):
      is_published = models.NullBooleanField(default=False)
      name = models.CharField(max_length=255)
  </code></pre>
  
  <p>I registered the model in admin.py</p>
  
  <pre><code>@admin.register(College)
  class CollegeAdmin(admin.ModelAdmin):
      list_display = ('id', 'name')
  </code></pre>
  
  <p>Now I want to have a button on the admin panel, on pressing which I can change the <code>is_published</code> field of all the objects in the model to <code>True</code>.</p>
  
  <p>I have no idea how should I proceed. Any working code snippet will be appreciated.</p>