Replacing garage door springs can be very dangerous if you are unsure of how to do it. You will first need to determine exactly what kind and length of spring you will need. Check the owner's manual for your door to find that information. Removing the old springs and putting on new ones requires at least three people. Two people lift the garage door to release the tension on the springs, and the third person removes the old springs and replaces them with the new ones. Remember that your garage door springs are under incredible tension, even at rest. So be very careful when taking off the old springs and putting on the new ones.