code: func (s *Store) Update(key interface{}, data interface{}) error {
  	return s.Bolt().Update(func(tx *bolt.Tx) error {
  		return s.TxUpdate(tx, key, data)
  	})
  }
docs: // Update updates an existing record in the bolthold
  // if the Key doesn't already exist in the store, then it fails with ErrNotFound