Most of the time? the frame is in good condition and it's the fabric that gets ripped or worn?and that's an easy fix.
First?remove the cushion and any padded areas with a screwdriver.
Spray paint the chair- frame with an indoor/outdoor semi-gloss paint. One can will give you enough for two coats. Let it dry completely in between the coats. You may have to use a primer if your going to a lighter color.
Pick up some fabric to match the frame? You'll need enough for your seat pad, plus 6 inches to the width and length of the cushion. If you need new foam make it a ½ inch bigger than the seat.
Layout the new fabric, aligning any pattern, add any foam?and place the seat cushion face down? Trim the fabric out leaving a three inch overhang all the way around the cushion.
Attach the fabric using a staple gun. Begin securing the centers of each side and then the corners.
Once the chair paint is dry, replace the new cushion?if you have a full metal back you can even customize it with some vinyl stickers? I'm Mrs. Fixit and it's just that simple!