When I start my project, the code starts with a Constraints Layout. How do I change that to the relative layout?
You can delete everything in your layout file and replace it with the new code that you are adding in the book. You do not need to change it to RelativeLayout.
The new project template has changed, but the code that you write will not.
Is there any reason (other than simplicity) not to use Constraints Layout?
As you work through the book and learn, I strongly recommend that you stick with the route as described in the book. That way, you will learn as you go with the guidance from the book. There is a chapter later on dedicated to ConstraintLayout.
As for a more general answer (once you’re done with the book): Google is pushing hard on ConstraintLayout. They want it to be the standard UI tool for most Android developers. I hope that this will happen one day, but in my opinion we are not yet there. The tooling in Android Studio for creating UIs graphically is lacking and ConstraintLayout has some performance issues. Google is constantly improving both of these so we may see a future where ConstraintLayout is the way to go. I recommend that you try out all of the tools that are available to you (you will throughout the book) and then you will be in a good place to evaluate which tools work for your situation.
A very reasonable suggestion
Google is pushing hard on ConstraintLayout. They want it to be the standard UI tool for most Android developers.