So are ATM machines monolithic devices where you have to scrap the entire thing if you want to upgrade the OS? Or do you open a door in the side, remove the old

So are ATM machines monolithic devices where you have to scrap the entire thing if you want to upgrade the OS? Or do you open a door in the side, remove the old computer and hook up a new one, and as long as you have the drivers that same cash dispensing and receipt printing hardware will continue working?