Other (generally older) doors have longitudinal springs. When the door is open, the spring is uncompressed, and very easy to change. Just make sure to block the door so it can't close while you're working, and install the safety wire (it goes through the spring so if they spring breaks, it doesn't hit something).

Modern doors usually don't have this kind because they take up space in the garage. The torsion kind are on top of the door and stay out of the way.