title: Changing label's text in another view controller
body: <p>I have one view controller named FirstViewController, and a second named SecondViewController. I present second view controller with </p>
  
  <pre><code> UIViewController *controller = [self.storyboard instantiateViewControllerWithIdentifier:@"mainController"];
  [self presentViewController:controller animated:YES completion:nil];
  </code></pre>
  
  <p>In SecondViewController's .m, I want to change the text of a UILabel in FirstViewController. However, the label's text isn't updating. How would I make it so that the FirstViewController's label is updated when a UIButton is pressed in SecondViewController?</p>