Make sure the app has access (permissions) to your external storage.
From within the app, open the corresponding torrent, tap on the file you would like to move. Tap on the three dots icon at the right hand top corner of the app, then tap on move torrent. You will then be able to see the torrent location and move it to a destination of your preference.
If this does not work, one workaround is to download the torrent to your phone, then use a free file manager app on the Google Play Store to move the file around.