normally it gets set once and can't be changed, either at the factory or when it first starts up. some will let you change it once or twice. if it is locked and won't let you change it, your only option would be to flash the firmware and set it then.
not sure how hard it will be to find firmware but i have done this on all kinds of devices like this. portable dvd players, home dvd players, even a tv that had a built in dvd player once.
you will need to do some serious google searching with your model number but in the end i normally find the firmware i need. if it is a popular brand you may even find custom firmware with new features available. have had this happen a few times but only with popular brands like sony and samsung