If you can't find the controls to change for shutter speed, ISO, or white balance, you're probably in "Automatic" mode. You can tell by the triangle on the top of the screen.
In automatic mode, you'll see an "EV" button on screen. It's in the upper right on iPhone X, and lower right on classic iPhones.
Tap on the Triangle to enter Manual mode. Buttons for ISO and White Balance will appear.
In Manual mode, EV will change to a shutter reading. Swiping up and down adjusts shutter speed.
What about Aperture?
Unfortunately, it's impossible to change the camera aperture because all iPhones have fixed apertures. However, if Apple ever releases an iPhone with adjustable apertures, we will support it in Halide.