To change propane fireplaces to natural gas requires that you purchase a conversion kit from the company that manufactured your fireplace. The kit comes with fittings and the flow regulator parts, gaskets and compression springs you will need to modify the fireplace regulator unit.
Cut off the gas supply to the fireplace. Typically, this can be done with a simple shutoff valve, lever or faucet-like affair not far from the fireplace.
Find the regulator for your fireplace. It will be attached to the gas pipeline between the feeder pipe and the burner assembly. Check your fireplace owner's manual for instructions for opening the panels protecting the regulator assembly.
Loosen the fittings and disconnect the regulator from the gas supply line.
Find the orifice on the regulator. Natural gas and propane have different densities and therefore must be supplied in different amounts to the fireplace. Gas delivery is controlled by the size of the orifice. The orifice looks like a short, fat bolt and has small hole in the middle of the bolt head.
Unscrew the old propane orifice.
Wrap the threads of the new orifice with plumber's pipe tape and reinstall it in the regulator assembly.
Wrap the gas supply pipes with pipe tape and reattach the regulator assembly. Reinstall the protective plates over the regulator and reopen the gas supply line.