1998 Ford Mustang check enging light won't go off

Mechanic ran diagnostics. A sensor needed to be replaced. Replaced the sensor. Mechanic said the light would go off after driving the car 50+ miles but it did not. Took the car back and diagnostic results were the same as before replacing sensor. A friend told me the computer may have to be reset and there was a way to do it. It involves unhooking and hooking the battery back up several times and turning the lights on and off. He wasn't sure exactly how to do it though. I'm not sure he knows what he's talking about.
I would like to know if resetting the computer could possibly be the answer, and how do I do it if it is?