code: def update(self, collection, selector, modifier, callback=None):
          
          self.call("/" + collection + "/update", [selector, modifier], callback=callback)
docs: Insert an item into a collection
  
          Arguments:
          collection - the collection to be modified
          selector - specifies which documents to modify
          modifier - Specifies how to modify the documents
  
          Keyword Arguments:
          callback - Optional. If present, called with an error object as the first argument and,
          if no error, the number of affected documents as the second.