middle heater knob need replacing?

The only model that uses that style control panel is the Equinox. To replace the control assembly, you remove the center trim panel. It is clipped to the dash with retainers, use a trim stick and the lift up at the bottom, once the lower retainers are out then pull out to disengage the others. Now remove the retainer screws and the control head comes out from behind. Reverse the process to install the replacement. To get the temperature control correct, center the knob on the front of the panel. Then look on the cable and find the centering tab as shown. Then plug it in so the gears mesh and the cable moves easily.
The part to replace it should be GM 19130174. However, that part has been discontinued so the only place you are liable to find an OE would be as a used part.