The photo taken by mobile phone is dark, how to solve it?

With the iPhone? 6s, iOS 12.3 as an example:

Tools needed: a photo, a gallery.

First, find the gallery on the desktop and open it.

Second, find the underexposed photos in the gallery and open them gently.

3. Click "Edit" in the upper right corner.

4. Select Adjust in the navigation bar, and then click Light Efficiency.

5. Slide the picture from right to left to get a photo with normal exposure. Click "Save" in the lower left corner.