Don a pair of latex gloves to protect you painting from skin oils. Orient the canvas so that you can access the back of it.
Clean away dirt, dust and other residue from the back of the canvas using a soft-bristled scrub brush.
Locate the dent or loose area of the canvas. Fill a plastic spray bottle with water and mist the damaged area lightly.
Allow the canvas to dry completely. Once the water evaporates, the canvas should be stretched taut again. If the canvas is still loose, proceed to the next step.
Coat the damaged area of the canvas with a light mist of canvas-tensioning solution and allow it to dry completely. This solution is available at most art supply retailers.
Place the canvas painted side down on several flattened towels. Wedge your flathead screwdriver under the staples that secure the canvas to the back of the stretchers and push down on the handle to pry them up. Continue until each staple has been removed and the canvas is loose from the stretchers.
Position the stretchers, beveled edge facing own, over the unpainted side of the canvas. Check for correct positioning by pulling the canvas up over the stretchers' edges.
Grasp the canvas at the center of one of the stretchers' edges and pull it taut. Put in the first securing staple while the canvas is tensioned. Add two more staples on either side of the first, roughly two inches apart.
Perform Step 3 for each edge of the stretchers. Continue stretching and stapling the canvas to the edges at two-inch intervals. Pull the canvas tighter as you move toward the corners of the stretchers.
Fold the canvas at each corner and put in two securing staples to finish the restretching process.