[Vicegrip]

I work on them all the time. Don't listen to the folks that make torsion springs seem like unstable explosive devices. You might find that this is not a business that has a high labor rate and you might be better off shopping the repair around a bit.
If you want to do it yourself you can go on the internetz and find sites that sell springs all ready to go and offer instructions on how to replace them. Once replaced they need to be wound and this is where many get nervous. Like many things the danger is not in doing it right but not being informed as to how to do it and following simple methods. And contrary to popular belief you can in face weld a spring back together. Weld one half coil around from the break in each direction using a 50% stitch weld. Rewind and done. I have had welded springs that have outlasted new springs cycle for cycle and when they did break it was not near the welded coils. I use welding as a temp fix only for the most part. In a house setup temp might mean 5 years additional service or a week.

The spring is measured 5* ways. 1. wire size. The diameter of the wire itself. 2. coil size. measured as an outside diameter. 3. length of the entire coil. 4* right or left hand. 5* jack shaft diameter. the size of the rod the spring is mounted on.
*Right or left and jack shaft dia only maters for finished springs. Some springs are sold raw, you install the ends the heat the wire ends red hot and bend in the hooks. I don't recommend this format if you are new to this.

How old is the setup? I ask as I tend to see springs fail one close to the other if the failure is due to high cycle numbers. If the door has been in service for 20,000 cycles you will do well to replace both springs at the same time. If the door is a year or two old in normal home use the failure might have been due to a defect in the wire or installation.

With the door pictured you would first count the number of paint spirals in the good spring and then de-tension it. With no tension the spiral you see will be a straight line painted or marked across the coil. Unbolt the 2 bolts in the middle support bracket and loosen the clamping bolts on the other end of the spring. lower the broken side down a bit and slide the bad spring off the shaft. Slide the new spring on and bolt everything back together. Mark the new spring with a line across the coils. Wind in the same number of spirals as the good side had into both springs and tighten the pinch bolts. each 360 degree turn of the spring end adds one spiral to the originally straight line. Test door and go have a satisfying beer.