code: func (a *API) UpdateContactGroup(cfg *ContactGroup) (*ContactGroup, error) {
  	if cfg == nil {
  		return nil, fmt.Errorf("Invalid contact group config [nil]")
  	}
  
  	groupCID := string(cfg.CID)
  
  	matched, err := regexp.MatchString(config.ContactGroupCIDRegex, groupCID)
  	if err != nil {
  		return nil, err
  	}
  	if !matched {
  		return nil, fmt.Errorf("Invalid contact group CID [%s]", groupCID)
  	}
  
  	jsonCfg, err := json.Marshal(cfg)
  	if err != nil {
  		return nil, err
  	}
  
  	if a.Debug {
  		a.Log.Printf("[DEBUG] update contact group, sending JSON: %s", string(jsonCfg))
  	}
  
  	result, err := a.Put(groupCID, jsonCfg)
  	if err != nil {
  		return nil, err
  	}
  
  	group := &ContactGroup{}
  	if err := json.Unmarshal(result, group); err != nil {
  		return nil, err
  	}
  
  	return group, nil
  }
docs: // UpdateContactGroup updates passed contact group.