Summarize:

Before you can work, you need to release the tension on the springs. Open the door until it hits the stop bolt and can't go any farther. For an automatic door: Open the door, unplug the opener, and disconnect the door from the opener by pulling the emergency release cord down and back until the spring locks in the open position. You will then be able to open the door fully and release the tension on the springs. Place a C-clamp below the bottom roller, or use two locking pliers (one on each track under the bottom rollers). This will ensure the door doesn't fall while you are working. The spring will be attached to the track hanger by a large hook that's kept in place with a nut. Once there's no more tension on the spring, remove it from the track hanger. Use the adjustable wrench to remove the nut on the back side of the bracket. You'll now be able to move the hook to a lower or higher hole to decrease or increase the tension. To make sure you achieve the right tension, adjust the spring by one hole at a time.Adjust both springs equally, one at a time, if your door was balanced. Once you've moved the hook to its new location, replace the nut on the back and use the wrench to tighten the hook into place.   To decrease tension and fix a door that isn't closing fully, is difficult to close, or opens too quickly, hook the spring onto a lower hole on the track hanger. This reduces the length of the spring and the tension on it.   To increase tension and fix a door that is difficult to open or closes too quickly, hook the spring onto a higher hole on the track hanger. This stretches the spring and increases the tension on it.   To adjust tension on one side and correct a door that wasn't closing evenly, adjust the spring only on the side where the gap was. Decrease the tension on that side by hooking the spring onto a lower hole on the track hanger. After adjusting your spring(s) by one hole, unclamp the door and test the springs by lowering the door to see how it feels. Make another single-hole adjustment if your door is still closing too quickly, slowly, or unevenly (with gaps). Repeat steps one to five until your door is closing properly and evenly, or until your gap is gone. The cable inside the spring will also be attached to a hook or the track, and you can adjust this by tightening or loosening the knot or the clamp attaching it and tweaking the length of the cable.  Shorten the cable to slightly increase the tension, or lengthen it to slightly decrease the tension.  Your cable may be attached to the track by an S-hook, and if that's the case, move the hook to a higher hole to increase tension slightly or to a lower hole to decrease tension slightly. Determine if your door is opening and closing the way you would like, and ensure it's balanced (closes evenly and without gaps). Continue making minor adjustments with the cable and testing the door until it's correct. When you have finished with your adjustments, be sure to remove anything that was securing the door in place, pull on the emergency cord to release the spring and reattach the door to the opener, and plug in your automatic door opener.
Open your garage door fully. Secure the door in place. Remove the spring hook. Adjust the tension. Test your door for functionality and balance. Adjust the cable to make minor tension adjustments. Test the door.