You could change the fluid if you want but do it at your own risk- it could damage if performed incorrectly. You said it shifts slow, it's got about a quarter of a million miles- it's worn out. Just drive normally and don't drive like a maniac otherwise your transmission will go out.

Perform regular oil changes and keep a watchful eye on your fluid levels and all of your radiator hoses. Check your heater core hoses, people tend to forget about those when they replace their radiators and the big radiator hoses.

As far as the transmission fluid, changing it is risky. If it's red, not brown like varnish, and doesn't smell burned, then it might not hurt to change it. Do that at your own risk. You might also consider changing the filter and keeping the current fluid.

I have a 1999 3.0 Ranger with 269k and it's only gone through a half quart of oil in 4,000 miles. All of its previous owners took very good care of the engine but forgot about the transmission and it was pretty shot at 249k when I bought it. It was 100% the original transmission. I had it rebuilt.  Recently a heater core hose ruptured at a plastic T-joint when I was a mile away from home, dumping over a gallon of coolant in that short distance. I make it a habit to glance at my vehicles when I walk away since two of them are old, and by doing that, I noticed the antifreeze puddle right away. Pick up a couple of my habits, and that Ranger should last a while longer.