Inspect each point where the chair fabric is attached to the frame, and wherever two of the supporting tubes meet. There should be a fastener at each of these points. Jiggle the connection to see if the fastener is loose. If it is, turn the screw with a screwdriver, or bolt with wrench until the connection is tight.
Replace any missing hardware with parts from your local hardware store. If you have trouble finding a specific piece of hardware, there are online businesses that specialize in replacement hardware for outdoor furniture.
Apply a high-quality fabric patch if the chair fabric is torn or has a hole. Instructions for various patches differ---please refer to the manufacturer's directions. If the fabric is detached from the frame, use caution as a patch may not be strong enough to support your weight.
Replace the entire fabric seat if it is badly damaged. Remove the old seat by unfastening the connecting hardware from the chair frame with a wrench or screwdriver. Set aside the hardware for use with the new seat. Once the fabric is detached from the chair, trace its shape onto your replacement fabric with a permanent marker. Cut out your new seat along the marked line with scissors. Sew the fabric binding to the cut edges. Reassemble your chair.