Most of the time, canvas can be machine washed and then put back on the furniture while it is damp to stretch the material back into shape. However, if you can't remove the canvas, use a scrub brush and rub it across the bar of Fels-Naptha soap and then rub the brush back and forth on the canvas. You may have to do this a few times — don't be afraid to really lather it up. Fels-Naptha soap works well for canvas awnings. It does a good job removing bird droppings and many stains.
Once you get the canvas nice and clean, take the hose or a spray bottle and rinse it off.