I need a new computter in my car

I have a 95 grand prix 3.1 the service engine light keeps coming on especially when I speed up then slow down I took it to auto zone for a free diagnostic check and the meter they use wouldnt fit the computer in my car so he got a special key to try it and no codes came up he said the computer is no good but I would like to try to replace it my self I am a female and have only dealt with small engine repairs but would like to try to fix this myself I just need to know where the computer is and how do I take it out to put a new one in? Thank you for any help you can give me on this problem