Pull the lever, if possible, to raise the footrest all the way. Place an object under the footrest to prevent it from moving up or down. If you can't get the footrest up because the arm is too loose, turn the sofa upside down.
Pry off one or two wood or metal caps located around the perimeter of the arm where it attaches to the side of the sofa. There is one cap on each side of the arm. Use the tip of a screwdriver to pop the caps off to reveal screws or bolts. If there are no caps, locate a single hole in the bottom of the arm; it does not have a cap.
Tighten the screws securely using a screwdriver. If there are no screws, use a 3/8-inch socket and ratchet to tighten bolts inside the holes. Insert the caps back in the holes. Tap them down tight with the back of the screwdriver.
Remove the objects from under the footrest or tip the sofa upright.