It is quite unusual that you would need to change the tension in an overhead door garage spring. It may have slipped over the years or the spring has stretched or, in the worst case, snapped, which would require replacement. Springs should be adjusted only so that the door lifts easily and doesn't fall back to the ground. If the tension is too tight it will be difficult to close. A single wooden garage door can weigh over 200 pounds.
Find the two 3/8-inch holes on the right side of the spring. Insert one of the 24-inch pieces of bar stock in the hole so that the bar is pointing downward. Using the appropriate size open-ended wrench and holding onto the bar tightly, loosen the two nuts and let the bar rotate backward so that it is tight up against the garage door. At this point you can take a break and plan your strategy for breaks and testing.
Insert the second bar in the other 3/8-inch hole. Push this bar toward the ceiling of the garage. Holding the second bar, push this toward the ceiling. Remove the the top bar and replace it in the bottom hole. Let it rotate back so that it rests on the garage door so the bars don't slip. Tighten the nuts with open-ended wrench.
Remove the bars. Try opening and closing the door a few times. Let your kids or wife try it and if it is an easy lift and shut, the job is done. If it needs further adjustment repeat the above steps.
This is an extremely dangerous repair. Wear gloves and safety goggles and be very methodical. If the bar slips, your face will be the target and you are likely to fall off the ladder.