My son picked up one that someone had put out on the curb, We disassembled the heater to access the valve. Typically there are 2 Phillips screws (one on either side of the valve stem). Once we removed the screws, the complete valve assembly pulled apart.
I pulled the valve stem out with a bit struggle using a vice grip pliers. We lubricated the valve stem with white lithium grease and reassembled the valve. I then worked the valve on and off to spread the grease and loosen the valve so that it traveled easily.
We reassembled the heater and tested and it worked as it should.
The big box store patio heaters are cheap Made in China junk and the valves seize due to galvanic corrosion - brass/aluminum.
Total time spent an hour.