Find your starter and solenoid. On your 240SX, it is located on the driver’s side next to the bell housing of the transmission.
Remove the nuts holding the two wire connectors on the solenoid. Mark the two wires with tape so that you connect them to the correct terminals on the new solenoid. Also remove the electric wiring harness, by hand, from the back of the solenoid.
Remove the nut that mounts the solenoid to your starter and pull off the old solenoid. Set the new solenoid in its place and replace the holding nut with your wrench.
Plug the wiring harness into the back of the new solenoid and then re-connect the two solenoid wires. Replace your Nissan’s negative battery cable and then lower the vehicle down from the jack stands.