Garage door springs can be a very dangerous home maintenance project for a person to undertake. The springs on garage doors have an extreme amount of pressure on them. If they don’t have a safety cable in the middle when they break, one can literally go through a wall. To avoid this danger, it is best to let a repairman fix the problem, but if that is not an option it can be done with a few precautions.
Raise the garage door so that there will be the least amount of tension on the springs.
Replace the broken spring by copying the way the other spring is attached. Make sure that the sheaves are not stretched more than an inch apart while the door is up. The spring will stretch out as the door goes down, and if it is stretched too far before the door is lowered, it could break when the door is down. If the garage door is still heavy when installation is over, it can be undone and the spring can be stretched a little more as long as you make sure the sheaves are not stretched more than a couple inches max when the door is up.