Place a towel down on a table or floor to create a soft surface where the painting can be placed while removing the staples.
Lay the canvas down on the towel if the staples are in the back. If the staples are on the side, place the canvas on its side and work on one side at a time.
Pry your staple remover or flat-head screwdriver gently under the first staple. Upholstery staple removers have a forked point. If the staple is tightly adjoined to the canvas and wood, you may need to "dig" under the staple in order to get the staple remover under the staple. The wood used in stretcher bars is soft, so pushing down under the staple should not be a problem.
Push the handle of the staple remover/screwdriver down carefully to help loosen the staple. If the staple is tight, lightly tap the handle of the remover with a rubber mallet.
Use needle-nose pliers to pull the staple out completely.
Remove all of the staples in this way until the canvas can be lifted off the stretcher bars.