It's quite easy to do. There's a plug underneath, at the bottom of the case, you drain the fluid, put the plug back in, remove another plug a little further up on the tranny case and fill with gear oil just until it's level with that upper plug hole, then put the plug back in. It'll take about 2.2 litres to fill it. If you drive the car hard i'd say to change your fluid a couple times a year, or maybe even after every HPDE you do if you use Honda MTF, and once a year if you drive casually. You can't be too careful with your tranny, it seems many people overlook changing the fluid often.