Mask it out, replace it., apply blue toning to the rest of the shot and decrease contrast and saturation. Enough tutorials out there.
Day for night should be back lit. You need to desaturate colors as much as possible, crush blacks and put a sharp shoulder on the whites. Sometimes adding a blue solid above the footage with the blend mode set to multiply helps. If the footage isn't shot with day for night in mind it's really difficult to pull off a day for night effect.
It would be a lot easier to give you some direction if we had a screen shot or two of the shots you are trying to use.
Thanks for heads up guys. I love the net...http://library.creativecow.net/articles/kramer_andrew/Day_to_Night/video-tutorial
This was a great Tutorial for that question. For those looking too. Guess I shoulda Googled before Posting....