A/T Transmission Help need ?!?!?

I own two Geo Storms. One of them me my dad, and my brother-in-law tried to get runing but couldn't. So I had bought a 91 Storm Gsi. When I had bought it the man said that the car was stuck in third gear. With it in that gear the car doesn't shift down or anything. I will start and drive fine. It is just slow on accel. until that gear is need. I just need to know how do I get the car to shift again....