Tap the white arrow at the top to bring up manual controls. Tap the exposure icon (+- signs in a circle ... If you have an iPhone with a Home button, such as an iPhone 6, 7, 8, or an iPhone SE 2 or ...
Use a third-party app to manage duplicate pictures if you have a lower version and don't want to go through the time-consuming manual process. These applications scan your iPhone for duplicate photos.
With a geomagnetic storm incoming after the most powerful solar flare of the current solar cycle, here's how to image the Northern Lights with a phone and manual camera.