If your Mercury Mystique has the OD light flashing and the car seems stuck in second does it need the fluid replace or a new transmission?
IF the transmission is slipping when it shifts, I would say it's too late for changing the fluid. I have been in touch with many people who have had these trannies go out. I've had two that first starting slipping when they shifted from second to third gear.I'm in the process of taking one apart to see if I can learn what is causing it to fail. I wouldn't spend very much on an overhaul as they aren't worth it.
The easy way is to go to a marine supply house and purchase an engine oil suction pump. The hard way is to drop the transmission oil pan. If you do this you can change the minimal filter that is present. This is not recommended unless you have a lift and the proper tools including a transmission drain pan.