Darkening background?

How do you darken the background (everything excepth the car) to make it look like the night? I know you have to select it first, but then what?

mmm.... you could try putting a black layer on it, bu then you would have to used coloured flares for the light for any realism, and keep reflective things the same colour.

Copy Everything you dont want to darken to a new layer then working on the background adjust the brightness

aha there you go.