Levels

If you want to adjust the brightness or the contrast of the whole image, press "Levels" on the edit page.

The Brightness and Contrast parameters use "curves", so are less destructive than the more common linear versions of those effects than with simpler editing programs. The "Gamma" parameter alters brightness and contrast using a special "exponential" curve.

My advice: play around with them and see what works!

If you want to only alter the lighting of just the dark or light parts of the image, try the dynamic effects instead [Dynamic range] or [Shadows and highlights]

The "Clip dark" and "Clip light" are linear and remove light or dark colours. Use with caution (they're currently hidden only in the "advanced" menu).