Ext.define('TestPanel', {
  extend: 'Ext.panel.Panel',
  alias: 'widget.testPanel',
id: 'testerPanel',
title: 'oldTilte',
//other code
});

var grid = Ext.getCmp('testerPanel');
grid.setTitle('newTitle'); //doesn't update the title

//tried with including ref or view in the controller
this.getTestPanel().setTitle('newTitle'); //this didn't update it either