Here is the manual for the Rebel XT
http://helene.ethz.ch/edu/vp/instrumente鈥?/a>
When in the manual mode or M on the mode dial at the top of your camera, use the thumb dial at the top of your camera to change the shutter speed, to change the apeature, hold down AV +/- on the back of your camera (near the top right of the LCD Screen, just above the self timer button).
Just move the mode dial to change the shutter speed while in Av or Tv.
I would suggest reading the manual.
http://helene.ethz.ch/edu/vp/instrumente鈥?/a>