Can I adjust the exposure time (shutter speed) in Lumia Camera?

Yes, you can. In Lumia Camera, tap the shutter speed icon in the indicator bar at the top of the screen (2nd icon from the right). To adjust the exposure time, move your finger up or down on the slider.

Note: The max shutter speed is 4 seconds for Pureview devices, for other devices the max shutter speed is 1 second.