Selector knob?

I have a 2000 Silverado that the selector knob will not change from the A/C blower vents to the Defroster or Floor for heater. The blower and A/C work fine, but the selector will not change from vents to defrost or any other selections, what can I do tho fix this?
Thanks for any and All help,
and A very Happy New Year To You All!!

If I understand correctly, you are unable to switch from floor, bi-level, and defrost on the vents. I'm not positive anymore, but these use to be controlled by a vaccum system with doors that open and close to shift the airflow. So, there is a vacuum leak most likely. The other option is that they have been cabled, and the cable is stuck in one position.

Just for fun try taking your HVAC Controls out of your dash and plug them back in again. Maybe it's a dirty connector.