Adjust gas valve
 

I need to lower the output of my gas valve on a 2 stage 7 yr old Lennox
furnace, I want to lower its temp rise or operating temp. The plate on
the valve has 2 torx type screws and a large flat head in the middle.
Does anybody know this basic adjustment, my installer never checked temp
rise and its 5f to much, and im not paying for a simple adjustment.