Set the camera to Av, M, or Tv mode
Turn the main dial to change the aperture
Use the quick control dial if your Canon model has one
Press the Av +/- button and turn the dial if required on some models
Check the aperture value on the LCD or viewfinder
In Manual mode, adjust aperture with the control dial assigned to aperture
In some EOS Rebel models, hold the Av button while turning the dial
In some mirrorless Canon models, use the control ring or touchscreen controls if enabled
If aperture does not change, switch out of Auto mode
Refer to the camera manual for model-specific controls