Camera Help!!?

I just bought a Cannon EOS Rebel K2 35mm camera for a film cl. Our first ignment consist of us taking pictures in Manual Exposure Mode. We have to be able to change the shutter speed and the aperture. I figured out how to change the shutter speed but I can not figure out how to chnage the aperture manually! I have to take normal, under, and over exposed photos! Can anyone help me???