How to fix air flow from a/c or heat to where it blows to windshield and floor board.

buried somewhere in the dash is an inoperative blend door actuator. You should be able to isolate which one it is by what is not working, the ones that redirect air from windshield to floor are often not that hard to get to, but may involve going through the glove box opening, air bag opening, or just reaching way up under there. you'll have to get ahold of a duct diagram for your make/model car.

First of all, before ripping all apart, is the knob functioning properly and "feels" the same as usual?

The knob seems to be the same. Even when you turn to defrost you can hear compressor kick on. Just don't have the air flow to the windshield.

No air flow to the windshield

My a/c button to recirculate went out , it keeps on tapping away , does anybody have that piece that recirculates . Its a 2004 Nissan Sentra with 1.8