NOTE: My chairs are dining room chairs not patio chairs.
Since the chairs were not created in one piece, they have to come apart somewhere! Look on the back rails, side supports for the seats, etc. until you find the screws. They may have wood caps (look like bumps) over them which you have to gently pry off. Then the upholstery is easy. Wrap fabric around the top/seat, stretch it tight, miter the corners and staple it where it won't show! On the backs you might have to put another full piece over the back of the backs to cover the raw edges. Then, put them back on the frames the way you took them off.