K51A Transaxle Fluid Change

I have a 2000 John Deere Sabre 14538 mower with a K51A hydrostatic drive transaxle. For 16 years now I have serviced this mower yearly and made the necessary repairs to keep it running like new. I was always content that John Deere said the "transmission was not servicable" and there was nothing to do to it. I have had now trouble with this machine at all with cutting my lawn and then plowing snow with it in the winter. But recently I read that the fluid in this transaxle should indeed be change regularly. I'm aghast that I have done absolutely nothing with this and it still is working just fine. I'm now looking for a video to show me how to do this or for some advice. Pulling the transaxle is not the issue but I just want to make sure I service this correctly to keep this working properly.

Thank you so much for taking your time to read and respond to my question here.

Lisa