Since you already have PM, just download from the Channel D site. Your code will transfer automatically.
You can also go to Pure Music > About Pure Music and it should ask if you want to DL the latest version.
Once you do that, delete the PM Icon from the desktop and the PM PDF of the directions so you don't get confused on version history.
Then open the download (Finder > Pure Music 1.88) and drag the PDF to the desktop and the Pure Music icon back to the desktop.
Open from the icon as you normally do and click on Pure Music > About Pure Music and check the version. It should read 1.88.