First create your night texture using the original texture as a guide.
Then open the building in Gmax, open the material editor and select the one covering the building. Then add your new texture to the 'ambient' field of the material (you'll need to click the little padlock icon first before you can add a map to it) and re-export it.
Edited by TechnicolorYawn, 20 January 2007 - 07:59 PM.