Once you've added your own custom tuning and saved it to your Fender Connect account, you'll see it displayed under My Tunings in the Settings menu.
Simply swipe to the left on the tuning you'd like to delete, you'll then see a “Delete” button, which you can tap to delete the tuning.
Editing custom tunings is not currently available.
Deleting Custom Tunings is not yet supported on Android, but will come soon.