code: def update_channels(cls, installation_id, channels_to_add=set(),
                          channels_to_remove=set(), **kw):
          
          installation_url = cls._get_installation_url(installation_id)
          current_config = cls.GET(installation_url)
  
          new_channels = list(set(current_config['channels']).union(channels_to_add).difference(channels_to_remove))
  
          cls.PUT(installation_url, channels=new_channels)
docs: Allow an application to manually subscribe or unsubscribe an
          installation to a certain push channel in a unified operation.
  
          this is based on:
          https://www.parse.com/docs/rest#installations-updating
  
          installation_id: the installation id you'd like to add a channel to
          channels_to_add: the name of the channel you'd like to subscribe the user to
          channels_to_remove: the name of the channel you'd like to unsubscribe the user from