Turn the chair upside down, locate the spring retainer bolts on the bottom of the chair using the flashlight, then unbolt the two retaining bolts on each side of the chair using the ratchet and socket.
Pull one end of the spring from the bottom of the chair, then unbolt the other end of the spring from the foot rest using the ratchet and socket.
Bolt the new spring onto the foot rest with the ratchet and socket, then position the other end of the spring on the bottom of the chair where it will mount. You may need the help of a friend to hold the spring in place.
Bolt the other end of the spring to the bottom of the chair using the ratchet and socket.