How to Change Input Source on IPhone

I have an electric guitar connected to my iPhone 6 with an "Apogee Jam." I want this to be the GarageBand input source to the "Guitar Amp" instrument, and listen to what I'm playing on my beats headphones.

When I plug it all in and turn on the "Monitor", one of two things happens: sometimes it does exactly what I want and the guitar is the input through the Apogee. Other times I'm not so lucky and GarageBand decides that the input isn't the Apogee, but rather is the microphone built into my headphones. If this happens I seem to be stuck and can't find any way to change the input source to the Apogee. From online research, I've seen buttons to change the input source when you are doing this on an IPad, but that button doesn't seem to exist on an iPhone.

Is there any way to select the input source on an iPhone?

Thanks!