Unless you are towing a boat or trailer regularly, or drive in stop and go traffic during hot weather a lot, changing the fluid, or doing a partial change, would not need to be done at such low intervals. IMO. 15k intervals are pretty short.
Use Mercon V rated fluid as it is based on synthetic oil, and can handle heat with less breakdown.
You can replace about 60% of the fluid by removing the splash shield on the drivers side, and draining the transmission. It has a square holed plug that accepts a 3/8" drive ratchet or extension. Remove that and the fluid will fall out. You should get around 6 quarts. I think capacity is around 10, so you are doing a good percentage change with just a drain and refill.
My suggestion would be closer to the 30k mile interval unless subject to the previously mentioned conditions. If your driving is highway, or low density slower speeds, it should be happy with that interval. Fluid that smells badly indicates overheating and need for change. Mercon has a petroleum odor more than previous ATFs, such as Dexron and Type F, but if it smells or stinks, a change is in order. You can get gallons at Wally of Valvoline that meets the FoMoCo standard and is priced a bit less than quart size containers. You will need a long funnel unless your aim is very good. Measure what drains, and replace with the same amount.