The online help system is a good resource for these kinds of "How do I..." questions. You can access it by clicking on the ? at the top right of PD's main winfdow or by pressing the F1 key.

If you enter track order in the search box, you'll see this page as one of the results, which tells you where to find the Reverse timeline track order setting.

Thanks for the reply. To be candid, the online help is not simple enough for an absolute beginner to navigate. I didn't even know the name of that which I wanted to change. There were no search results for track priority, so I would have had to post here even if I'd made the search, to learn what it is I was describing.

Quote When I opened my first project I was asked whether I wanted the tracks above or below to have priority during playback. I'd like to change this setting in my existing project, but I can't seem to find where to do so. I looked through preferences but didn't come up with anything.

If I understand correctly, you want to invert the priority of the tracks, in a finished project.
See the image.
You can also change tracks one by one by selecting them and dragging them up or down right on the timeline.