Drain and fill the transmission. I believe your car has a drain plug on the transmission pan. Do 2 or 3 drain and fills over a short period of time in order to get most of the old fluid out. Fluid gets dark in color over time, it is normal and if your transmission is working like it should you probably haven’t damaged anything. Drain and fill with the correct fluid as specified by your owners manual. Flushes are money makers but are not recommended by Toyota.