Turn off the antique stove and move it away from the wall to get at the gas regulator in the back of it.
Turn the gas off and disconnect the stove from the gas supply.
Unscrew the old regulator, the square device the gas line is screwed into, and replace it with a new natural gas and propane regulator. Tighten the regulator in place with a wrench.
Loosen the screws in the cap of the new regulator and remove it. Turn the dial from “Natural Gas” to “LP.” Screw the cap back in place.