How to access text boxes on a particular slide and change its text in Google Slides?

I am trying to access a text box on Google slides in order to change its text but every time I run the programme, it prints "textRange" on the alert box despite the fact that there's some text in the text box.

function updateTextBox() {
  var slides = SlidesApp.getActivePresentation().getSlides();
  var shapes = slides[0].getShapes();
  var text = shapes[0].getText();
  SlidesApp.getUi().alert(text);
}