2003 GMC C2500 Air Condition Switch

Which switch button, are you talking about? the climate control unit in your dash? or the pressure control switch Under the hood?

The climate control switch it doesn't turn to the different settings and I don't know how to get to the underside of it.

to gain access to your climate control, listen very closely, . You're dash is like a snap together toy, with your fingers grabbed the trim that goes around your instruments, Your radio and your climate control pull it straight out (you may need a butter knife to get underneath the trim), once you have removed the trim you will gain access to your climate control unit and your radio and everything else.