If you don't see controls to change exposure settings, you're probably in "Automatic" mode. You can tell by the triangle on the top of the screen.
Tap on the triangle and a panel will slide out, allowing you to enter manual mode.
What about Aperture?
All iPhones have fixed apertures, so any aperture effects you see in photos is accomplished in software. However, if Apple ever releases an iPhone with adjustable apertures, Halide will support it.