How do I fix the front defrost knob on this vehicle

I turn the knob on for the front defrost to come on and nothing
happens no heat will come from the dashboard vent for defrosting
the front windshield, but when it saying its on defrost. It blows heat
from the front vents and not the top or bottom. So how do you take
the knob off to fix the problem. what do you think the problem can
be and to fix it