code: func (kb dbKeybase) Update(name, oldpass, newpass string) error {
  	info, err := kb.Get(name)
  	if err != nil {
  		return err
  	}
  	switch info.(type) {
  	case localInfo:
  		linfo := info.(localInfo)
  		key, err := unarmorDecryptPrivKey(linfo.PrivKeyArmor, oldpass)
  		if err != nil {
  			return err
  		}
  		kb.writeLocalKey(key, name, newpass)
  		return nil
  	default:
  		return fmt.Errorf("Locally stored key required")
  	}
  }
docs: // Update changes the passphrase with which an already stored key is
  // encrypted.
  //
  // oldpass must be the current passphrase used for encryption,
  // newpass will be the only valid passphrase from this time forward.