In looking at your panel, it seems like it was hit with water at some point. I had a similar issue
This repair sort of comes under Murphy's Laws....go ahead and try it, if you break it you had to replace it anyway
Turn off the power to the panel
Remove SWG cord
Remove the 3 screws holding on the steel plate
Remove the display board (it sits on 4 nylon posts) and unplug (be sure to note orientation of wires, I don't think it was keyed)
Remove the screws holding in the pc board to the display unit and remove it
"button" pad will then be exposed, carefully peel back and look for discoloration/corrosion on the contacts
Clean carefully the pc board "contacts" with some q-tips dipped in alcohol
For heavier oxidation, try a pencil eraser but do not rub too hard - do not scrape with a sharp metal object or sandpaper
Clean rubber "button" pad with alcohol and let dry before reassembly
Power it back up and see how you made out
Here is a youtube video on cleaning the contacts for another piece of equipment to give you an idea how to clean the buttons, the principals are the same
How To Repair Remote Control Buttons That Dont Work - YouTube