How do you replace an o2 sensor on a Honda Accord EX and how much will it cost?

If you go to the dealer, it can cost upwards of $300 with a OBD II code lookup and diagnosis. Save some $50 on that by going to the local auto parts store first and having them pull the code. The code for the sensor malfunction is PO141... Anyway, going to say Tire Kingdom will be about $150 but you get an aftermarket sensor, and if the light comes on again, you'll be further in the hole because the symptoms of a bad O2 can be similar to other things, including but not excluded to a bad fuel filter, and many, many more. The best bet is to have the code pulled, call 3 shops, and ask for a rough quote... if you are within reasonable money to go to a dealer, do so.