The knock sensors are located on the lower part of the engine block, one on each side of the motor. The passenger side is located just in front of the starter, the drivers side is a little harder to see, kind of behind the a-arms. Make sure the car has cooled off when you remove them! I had to remove the starter to get to the passenger side one. These things are sometimes hard to remove because the outside part of the sensor detaches from the threaded part, making it impossible to turn.
I did the fluid change on a lift, it was tough even having the car in the air. I'd go to a place that has a coolant exchange machine, they can get most of the old coolant out without going through all of the hassle of removing the sensors. Bottom line, it's not a fun job, good luck if you attempt it. Narti.