look on the bottom of the engine, you will see this BIG flat head screw (or it could be a bolt on the A35 engines) thats the transmission fluid drain, ride the bike a bit 2 warm up the fluid, then, using pliers if its a screw or a wrench if its a bolt, so u dont burn the crap-ola outta ur hand on the muffler, loosen it (dont remove yet) get an old pan or some container that will slide under the bike (i use a smallish plastic tupperware storage bin) remove the bolt or screw, and let all the fluid run out in2 the container.
put that drain plug (be it a bolt or screw) back in & tighten it snugly (but dont strip it!). now remove your side panel on the transmission side (the side the chain sprocket is NOT on) and remove the big screw (or bolt) at the top of the transmission case, this is your transmission filler plug. its hole is rather small, so what i did, was to buy a small funnel and some 1/8inch tubing, cut a lil piece and secure it to the end of the funnel with duct tape, so it just slides right in2 the hole. On the side of the transmission case, near the bottom, theres a screw, remove it, thats your check screw for the fluid level, it makes sure you dont over fill your transmission. now fill your transmission with Dexron-III/Mercon ATF (you can get it almost anywhere they have automotive fluids, even kmart!) till it starts running out the hole of the screw in the side u just removed. wait till it stops dripping out, and replace that screw (remember to put it in sorta tight so it dosent leak or vibrate loose) then remove your funnel and reinstall the filler plug (screw or bolt) put your side cover back on, and voila! you just changed your transmission fluid!
hope this helps!
i usually change the fluid in my tomos (1990 TTLX, older style engine) about every 900miles or so, along with de-carbonizing the cylinder head and exhaust.