Repairing a garage door spring involves careful steps due to the high tension in the springs, which can be dangerous if not handled properly. Here is an overview of how to repair or replace a garage door torsion spring safely:

1. Safety First:
   • Unplug the garage door opener to prevent accidental activation.
   • If the door has a battery backup, disconnect the arm to fully disable the opener.
   • Use proper winding bars specifically designed for garage door springs; do not use makeshift tools.
2. Identify the Broken Spring:
   • Look for a visible gap in the spring or compare the springs if there are two.
   • Confirm by noticing that the broken spring will be separated or stretched differently.
3. Unwind the Tension on the Springs:
   • Use winding bars to carefully unwind the tension from the unbroken spring before removing any hardware on the broken spring.
   • Loosen the set screws on the winding cones using a wrench.
4. Remove the Broken Spring:
   • Loosen and remove the set screws that hold the spring to the torsion tube.
   • Slide the broken spring off the shaft carefully.
   • Remove any drums or cables attached to the old spring.
5. Measure and Prepare the New Spring:
   • Measure the length and diameter of the old spring for an exact replacement.
   • Springs have different wind directions (right-hand or left-hand), so be sure to match these correctly.
6. Install the New Spring:
   • Slide the new spring onto the torsion tube.
   • Reattach the drums and cables to the new spring.
   • Ensure the set screws are loosened on the new spring for winding.
7. Wind the New Spring:
   • Use the winding bars to wind the spring in quarter turns toward the ceiling.
   • Follow the manufacturer’s recommendations on the number of turns, typically 30 quarter turns for a 7-foot door.
   • After winding, slightly stretch the spring and tighten the set screws carefully.
8. Lubricate and Test:
   • Lubricate the springs with garage door lubricant.
   • Remove any clamps or locking pliers.
   • Manually lift the door about 3 feet; it should stay in place if tension is correct.
   • Adjust winding by adding or releasing quarter turns if needed.
   • Reconnect the door opener and test the door’s operation.

Garage door spring repair is labor-intensive and can be dangerous due to the tension involved, so proper tools and caution are essential. It's often recommended to hire a professional unless confident with the process and safety measures.