Remove the existing canvas. Use it as a template to cut fabric for a new seat, making the fabric 1 inch wider. Fold each long side of the fabric over 1/4 inch two times; hem.
Lightly sand the frame, and apply 2 coats of exterior paint; let dry. With a staple gun, attach one end of fabric to the underside of one of the frame's crossbars; repeat on other crossbar, stretching fabric taut.