1) Turn off the power to the Furnace unit at the circuit breaker.
2) Remove the cover panel on the HVAC unit that houses the limit switch. This panel is usually located on the front of the unit and can be removed with a screwdriver.
3) Locate the limit switch. It is a small device with wires connected to it. Take a picture or make note of how the wires are connected before disconnecting them.
4) Disconnect the wires from the limit switch by loosening the screws holding them in place.
5) Remove the old limit switch by unscrewing it from its bracket. Make note of how the switch is positioned so you can install the new switch in the same orientation.
6) Install the new limit switch in the same position as the old one, and secure it to the bracket with screws.
7) Reconnect the wires to the new limit switch and tighten the screws to secure them in place.
8) Replace the cover panel on the HVAC unit.
10) Turn the power back on at the circuit breaker.
11) Test the Furnace to make sure the limit switch is working properly.
Again, it is highly recommended to have a qualified HVAC technician perform this task to ensure safety and proper installation.