the P (Program Auto Exposure) mode is suited to those of you trying to break out of the Full Auto/a+ modes, but not confident enough to know which aperture or shutter speed to set. In P mode your EOS DSLR pairs aperture and shutter speed, but you can adjust them by using Program Shift. If you want a wider aperture or faster shutter speed, turn the Main dial to the right. If you want a narrower aperture or slower shutter speed, turn the Main dial to the left. You can also control other settings; some pro photographers say ‘P is for Professional’ and rely on it when they don’t have time to set an exposure properly to ensure they don’t miss a key shot.
You can override the aperture and shutter speed in Program AE