There are two different types of garage door cables- torsion cables and extension cables. To release tension on a torsion cable, you'll need to adjust the springs on either side of the door. To release tension on an extension cable, you'll need to remove the pulley from the wall and lower the door manually. Both types of cables are under a lot of tension and can be dangerous to work with if you don't know what you're doing. If you're not sure which type of cable your garage door has, it's best to call a professional to take a look.