They're either pop out plastic bits or there may be a couple screws. Most I've seen are just plastic and popped in to the holes with little built in tabs.
You'll have to fab up a blanking plate for the hole.
I would recommend you don't take them out. They're required for air flow in the cab and your blower motor isn't going to be as effective when the windows are up. They also help control humidity that accumulates in the cab from breathing and temperature changes.
Why do you want to remove them?
Lots of bass making them rattle or something similar?