Contact the heater's manufacturer to see if a conversion kit is available for purchase. The manufacturer might offer a kit complete with all the necessary parts and instructions.
If the manufacturer doesn't, purchase a kit or the required parts from a home improvement store.
Turn off the gas supply to the heater, and unplug any electricity sources to the unit.
Remove any panels, lids or other covers that protects the gas valves, orifices, burners or the pilot light.
Replace each part with the corresponding part designed for LP gas. Ensure that everything is properly and tightly secured.
Check for gas leaks. Prepare a solution of soap and water, and apply a small amount to the gas line connections.
Turn on the gas supply to the heater, but not any electric or burners. If bubbles form around the connection, it needs to be tightened.