I take for granted , you have the NAT gas regulator cap and the NAT gas burner oraface.

It was easier to remove the gas valve assembly(for me) but it's do able with the valve assembly remaining mounted to the base.

A) Unplug the power cord
Aa) Turn off the gas supply to the unit.
B) Remove the Brass "cap" screw from the top right side(hump) of the valve assembly( it should have a plastic plunger on the inside) and install the NAT gas regulator cap(no plunger)
C) Disconnect the wiring harness from the glow bar igniter and the radiant sensor(black box on side of burner housing)
D) Slowly and carefully, remove the screw securing the burner tube to the mounting plate(especially if you did not remove the burner valve assembly from the unit).
E) Using a 3/8 in. wrench or socket, remove the brass LP oraface and install the Nat gas oraface.

*** Be very careful with the glow bar igniter, it's very delicate, you don't want to crack or break it ***

Reverse procedure and re check burner operation/ignition.

Good Luck.