How do i dismantle dash-board

The problem is with the mode door actuator. See more on link below. 1997 Chevy Lumina Mode Door Actuator Replacement Air Conditioning

A vent flap actuator probably needs to be replaced. It's what moves your vent flaps from heat to A/C or to plain "vent air" when you change settings.

The discussion in the link includes vent flap actuators. Unfortunately, they also include the infamous no low speed fan problem too.

Cannot get any air from dash vents while defrost and floor vents work fine...

Your particular actuator will be a different one than they found.
They're all a absolute pain to get at because they can be well buried in the dash. I would start looking under the dash on the passenger side, that's where most of the air control is. Cannot get any air from dash vents while defrost and floor vents work fine...

In side you dash is a electronically controlled actuator call a mode actuator , it controls which vents the air comes out of ! They go bad, plastic gears in side of it wear out, a lot of times you'll hear a clicking sound in the dash. They are not to tough to fix ,you need to take Instrument Panel Insulator Panel off - Left Side under dash . Some times stuff drops down in an binds up the door inside .Have a little child , they like putting stuff in the vents ,pencil's an the like.
Mode Door Actuator Dorman 604 121 Controls Airflow Direction

There is actually two blend door actuator that control the opening and closing of your vents. Ine is located under the drivers side dash board. The is on the passengers side. Both are electric and I didnt have the money to replace them so i removed them and turned it with a pliers to allow heat to come out of bothe sides of the vent. Then come summer flip the little bar back the other way to A/C
Hope this helps
God blessss

check the air actuator door hoses your under dash vent doors are not working

The ducts have dampers that work off of a vacuum and control the flow of air to the different air vents at the top, bottom and face of the dash. Sometimes the vacuum lines come loose or break. You'll need to look under the dash to see if any of these conditions exist.

On a 1996 model you probably have vac actuators that control air flow. Look under the dash and you may be able to see the actuator that controls flow to the defrost and vents. There would be another one for temp control.

There is a door actuator motor that is not working,the dash will have to come out to replace the actuator motor.