Reprogramming a computer-controlled transmision?

[The Atomic Ass]

After quite a hiatus from even owning a vehicle, I've recently acquired the '96 Buick LeSabre my profile now sports. There is definite room for improvement with the transmission, as it doesn't even shift into overdrive until 45, or lock the TC until 55+, which is, in my opinion, not so good.

What options are available that will tune GM transmissions? This car specifically is sporting a 4T60e.