There's no switch for this, but it's possible in some images. If the exposure is off, tap on something in the frame that is about the same distance as your subject. Tapping something lighter will darken the image. Tapping something darker will lighten the images. In the example the exposure is thrown way dark by the light coming from the window. By tapping on the iPhone screen on the floor just in front of the window the focus and exposure are readjusted to lighten the interior.
If you're taking a shot of two people and one has on dark clothes and one has on light clothes, tap on the appropriate person's clothes to lighten or darken the image. It's important that what you tap on is the same distance from the camera, or the key parts of the image will be out of focus.
Camera: Adjusting Exposure
