Question: how to replace a garage door extension spring?
Answer: Unplug the power cord to your garage door opener. ... 
Disengage the opener from the door. ... 
Weigh your garage door. ... 
Shake the garage door to loosen the rollers in order to get an accurate weight reading. ... 
Since the extension spring on this door was broken on the left side, I weighed the door on the left side.