What sort of app is it? Audio or Nav?
If you have extra audio or navigation apps loaded on your phone you'll see a downward pointing arrow next to the icons at the bottom of the AA screen. Hold down (nav or audio) icons and up will pop a list of the things you can choose from.
I have TuneIn internet radio, BBC Radio IPlayer and Wayz installed and that's how you swap between them.
A bit not obvious!