I think you're talking about an O2 sensor , the front one is located on the pass. side exhaust pipe just before the two pipes come together , and just above the inboard CV
joint. follow the wires up to the connector , disconnect , and push the pig tail down
underneath the car,slide the connector through the boxend of a 22mm wrench and lossen the sensor.( a little penetrant helps). the rear one's a little further back looks the same , but isn't.