Just did this on my 2001 today. To get to the thermostat you need to remove air cleaner box assembly from the throttle body and move it out of the way. Next, remove the nut at the throttle body that holds the steel tube that goes to the heater hose, remove all hoses that connect to this tube and raise it out of the way. Remove the steel line that goes to the radiator surge tank from the thermostat housing. Now the fun part. With a 13MM box end wrench, loosen the lower bolt from the thermostat housing. A 13 mm socket will remove the top bolt. Remove the thermostat cover and remove the thermostat. Reverse to install.
I just did this on my 1999 Grand Am V6. The above information about removing the heater hose is absolutely necessary. In addition to thatb you will need to disassemble another electrical device that prevents you from being able to move the heater hose out of the way. Sorry I don't know the name of this device. It's located near the throttle body about 5 inches back towards the fire wall. It has 2 bolts which are easy to remove. Also you will need to remove the throttle body. Also I highly recommend that you remove the throttle body. If you don't remove the throttle body then you will be unable to rotate the thermostat housing, to be able to remove the thermostat. This also gives you extra room for getting to that lower bolt on the thermostat housing.
Overall, this is not an easy job due to the lower bolt holding in the thermostat housing, with minimal clearance to get a wrench on it. After you get it removed it is even harder to start threading that bolt for reassembly. You will need to fish around blindly, properly allinging the housing with the engine mount, while trying to start the thread on the bolt. The exaust manifold is in your way the entire time. If you have big hands forget about it. My medium size hands found this almost impossible. I would not do this job myself again.