I have changed the wheels and tires on my 2017 Chevrolet Express Passenger Van.
I want to adjust the tire pressure monitor threshold to 45 psi all round.
How can I do this ???

 

Not sure on the new vans but the older generations it could be done with a dealer scan tool in the ebcm.
even if it configurable, the dealer may not be willing to change this for you for legal or liability reasons.

 

Why do this? Factory psi ratings are for the van, not the tire. You may have new tires that say max psi is 50 but that does not mean you should inflate to 50. What tires did you get?