These screws are just for guidance and the burners are supposed to slide in and out underneath. They are normally rusted tight since the screw is not made of SS. Do not try to remove them. They will break off and then you have "nice" job on your hands drilling out the reminder of that screw.
If you wobble the burners a little they should slide out underneath. If noy you can take a flat head screw driver and and push the burner away from the left side wall.
I got lucky few times but other times no. I guess Weber did not expect for their grills to last this long. I replaced all burner screws with SS for the screws that came out, the screws that did not just slowly pulled the burner out. I then cleaned the area with solvent then put the new burner in.