If you bought the app directly from the Google Play store, simply reinstall it from the Play store.  First, make sure you are logged in to the same Google account that you bought the app with.  Next, on your Android phone or tablet, open the Google Play store app, then tap Menu, then tap My apps & games, then tap Library.  Tap the µTorrent app, then tap Install or Enable.

 

If you bought the upgrade through an in-app purchase, make sure you are logged in to the same Google account that you bought the app with.  Next, go download the free version from the Play store.  Once you open the app, it will upgrade itself to Pro automatically.