Text change on UIButton doesn't stick

In the button handler, try this:

[stopButton setTitle:@"RTN TO ZERO" forState:UIControlStateNormal];

Instead of directly changing text property of titleLabel use setTitle:forState: method to set the title in different states. Please check the manual for the details of available states.

Swift version

myButton.setTitle("button text", for: UIControl.State.normal)

Use setAttributedTitle:for for attributed text. See here for how to make attributed strings in Swift.