code: def update_label(self, old_label, new_label):
          
          logger.info("%s : Updating label ([%s] -> [%s])"
                      % (str(self), old_label.name, new_label.name))
          labels = self.labels
          try:
              labels.remove(old_label)
          except ValueError:
              # this document doesn't have this label
              return
  
          logger.info("%s : Updating label ([%s] -> [%s])"
                      % (str(self), old_label.name, new_label.name))
          labels.append(new_label)
          with self.fs.open(self.fs.join(self.path, self.LABEL_FILE), 'w') \
                  as file_desc:
              for label in labels:
                  file_desc.write("%s,%s\n" % (label.name,
                                               label.get_color_str()))
docs: Update a label
  
          Replace 'old_label' by 'new_label'