There is no drain plug; to change fluid you need to remove pan, dump fluid, take out magnet clean it up. New filter, new fluid. Drive 500/1000 miles, and change fluid and new filter because you can't drain the fluid from the torque converter when doing a flush.

Sometimes you may need to do three flushes + add some lucas transmission additive before it acts better.

But if its over or near 150,000 miles, don't bother just buy a used tranny and swap in.