code: func (ks *KeyStore) Update(a accounts.Account, passphrase, newPassphrase string) error {
  	a, key, err := ks.getDecryptedKey(a, passphrase)
  	if err != nil {
  		return err
  	}
  	return ks.storage.StoreKey(a.URL.Path, key, newPassphrase)
  }
docs: // Update changes the passphrase of an existing account.