Before you spend the big bucks on replacing converter(s), bear in mind that any exhaust leaks in the pipes or mufflers will also affect converter efficiency. So if there are exhaust leaks; sealing them all up may be enough to corrrect the problem without having to replace converter(s). Similarly; the code for a bad converter is generated by comparing the oxygen sensor readings in the sensors before, inside, and after the converter. If one or more of those 3 oxygen sensors has lost efficiency; that can trigger a converter fault code, even though the converters may still be good.
Replacing both converters will give the biggest improvement in emissions; but just replacing the front converter can sometimes be enough to prevent the trouble code from coming back; assuming the rear converter is not in very bad condition. Some Altimas with federal emissions were manufactured with only a front converter; so there would only be one converter to replace on those models. V-6 Altimas often have 3 converters (2 front and 1 rear). Federal and California converters and oxygen sensors are both different from each other. So you need to be very specific about the model year and month of production, plus the originally certified emissions type (Federal or California), in order to get the right parts.
After the exhaust work is done; if your car is an OBD II model (usually 1996 or later), it will be necessary to have the trouble code cleared with a code scanner. If the car is an OBD I model (1995 or earlier); the code can be cleared by disconnecting the battery for at least 60 seconds.