I have solved the issue by digging around in this forum's history. However I don't think it has ever been put together in a complete series of steps:
- bought a new 16GB Micro SD card at the store (it came with the SD adapter necessary to plug it into my PC.
downloaded Etcher (as described elsewhere in this forum)
downloaded the 1.0.6 Piper OS
flashed my new Micro SD card with the downloaded piper OS (-> NOTE: I had to append the .img suffix to the image file name in order for Etcher to recognize it as an OS image)
once the SD card flash was complete, inserted it into the Piper computer (with the contacts facing up) and booted up
connected a USB keyboard to the Piper computer
once booted all the way up, pressed the "power button" icon on the screen and chose "Exit to Desktop"
followed the instructions in the following thread to unlock the Story Mode levels my son had already completed:
[SOLUTION] To unlock the levels, please open the file manager and navigate to /home/pi/piper/userData. In the unlockedLevels.txt file, please change the number to 9. In addition, please navigate into the playData folder here and change the value in the highestLevelPlayed.txt file to 9. If you have any permissions errors, you can click on the update button (which will fix the permissions when finished) or you can open a terminal and type "sudo chown pi:pi -R piper/" followed by enter/return and then re-try the above edits.