Tap on the Camera Control icon (it is the fourth icon from the left).
The Camera Control menu will display the settings of the currently previewed source if the preview window is opened. If you do not have the preview window opened, the Camera Control menu will be for the source that is live. From here, you can adjust all of the camera settings like Focus, Exposure, and White Balance.
To adjust a setting, tap on the "M" next to it to change the mode from Automatic to Manual. A slider will appear on the camera that you can slide up and down to adjust the setting. Tapping on the lock icon will stop any adjustments from being made.
If you are using an iPhone with the horizontal UI, the process is very similar. Start by tapping the Camera Control icon in the lower left.
You can now swipe across to find the setting you are looking to adjust and change it from "A" to "M". When you are done, swipe down and the menu will disappear.