code: func (f *Function) RollbackVersion(version string) error {
  	f.Log.Info("rolling back")
  
  	alias, err := f.currentVersionAlias()
  	if err != nil {
  		return err
  	}
  
  	f.Log.Debugf("current version: %s", *alias.FunctionVersion)
  
  	if version == *alias.FunctionVersion {
  		return errors.New("Specified version currently deployed.")
  	}
  
  	f.Log.Infof("rollback to version: %s", version)
  
  	_, err = f.Service.UpdateAlias(&lambda.UpdateAliasInput{
  		FunctionName:    &f.FunctionName,
  		Name:            &f.Alias,
  		FunctionVersion: &version,
  	})
  
  	if err != nil {
  		return err
  	}
  
  	f.Log.WithField("current version", version).Info("function rolled back")
  
  	return nil
  }
docs: // RollbackVersion the function to the specified version.