Welcome to CEG! The pre-cats are very close to the engine and help clean up emmisions when the car is first started and the main-cat is still cold. If you pull the coil pack, you should be able to get an O2 sensor socket and breaker bar on the upper sensor. Others have used a stubby wrench and pushed on it with pipe. You will probably end up sitting on top of the engine so you can reach down to position the tool, but it's not bad once you figure out how to get it loose. Use anti-sieze on the threads of the new sensor.
