Where is and how do I replace the O2 sensor on a 1992 Honda Accord EX?

It's located in the exhaust manifold, between the radiator and the engine block. Look for a 4-wire pigtail coming out of the sensor. Here's how you can change the sensor yourself:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563.html#post223376