If there is a external filter you can go ahead to replace it. If there is none, then just drain out completely and replace the oil. You can also replace the oil again 1 or 2 weeks later for better flushing.
Please don't open up the gear box, it is not as easy task and not many mech out there can understand or work on it. If you ask them, they will say " Oh! can no problem, it is very easy we always do that", but later all problem come to you, your wallet and your time.....
Think twice......
