How should I adjust tire pressure to/from mountains

With 70-80 psi inside the tire, the less then 8 psi difference from sea level to 20,000 feet - amounts to nothing. Tire pressure on the sunny side of a parked car to the shade side will change the internal pressure more. At least monthly, (weekly towing) check the pressure before driving, and air up to your preferred pressure. Never let air out - it leaks out not in. Unless someone overinflated the tire you should never need to release air.