James O'Brien-3 wrote
  

SimpleButtonMorph's class comment, would the target be the TextMorph? If 
so, how is the correct TextMorph identified if there are two or more of 
them?

    

Yes, you'd need access to the TextMorph instance, and write something like:
button := SimpleButtonMorph newWithLabel: 'change text'.
button
	target: tm;
		actionSelector: #contents:;
		arguments: #('new text'); 
		openInWorld

If you didn't create the TextMorph and can't get access to it, you can find
it by searching through the world's submorphs (recursively if nec.)