Need to know if I can replace the knock sensor on a 2000 Nissan Altima as a DIY project & where is it located?

You should be able to do it. Locate the oil filter from under the car. Then look for the knock sensor, which is attached to the engine block slightly above and to the left of the oil filter. It's screwed into the engine block with a 12mm bolt. Look at what the sensor looks like first, then it should be easy to spot.