First, make sure you have both the free and paid versions of the app installed on the same device. Open the free version of the app and go to the Backups screen (Settings tab on iPhone or Actions toolbar button on iPad). Create a fresh manual backup by tapping Edit and then the + button that will appear. Tap the freshly created backup's icon (make sure you tap the icon, it is important). You should see a screen offering you to choose in which app to open the backup (see below). If it looks different, perhaps you didn't tap the icon, cancel and try again. Choose Open in Total Return and follow the prompts.