The transmission has a level check plug on the side of the transmission. Fluid is added through the vent until with the engine running fluid runs out the plug. Thew transmission takes Dexron (R) 6 fluid. Under normal conditions GM say change at 100,000 miles but I would change it at 50,000 if it were my car.
Changing the Transmission fluid is different for every car i suggest you look at your car's manual, if you have done every 2 years since you got the car you can keep doing it but if you haven't i wouldn't do it the transmission fluid has a lot of detergents powerful enough to mess things up in there.