it is at the rear of engine, you can seeit from under the car. it is bolted to the rear exhaust manifold, ..a bit out of sight in this picture
www.avigex.ca/xport/rearplugs.jpg you have to remove the exhaust shields (removed here) to get a better look. I think (not sure) you would need a special socket to unscrew the O2 sensor.
By the way, mention car model, engine type, model year at begining of all new threads. Not all will check your previous threads to find the relevant info.
ALso....what conditions tell you you have to change this O2 sensor....