When I bought my house, with its 12x12 wooden garage door and (at the time no other entry to the garage) I was sitting on my couch watching TV one night and BAMMM!!!! -- it felt like a bomb went off outside, Opened the door, looked outside -- saw nothing. The next day I go to open the door and it didn't want to go up. So I pulled the emergency (power outage) cord and tried to lift it. Got it to go up about a foot or so. Wedged something under the door and crawled in to find one spring had gone.
$750 later I had two new (special order) springs installed. I then worked to put another door into that door so I could go in and out without opening the big door. Well... that changed the weight of the door and the springs had to be adjusted.
Having done it at work many times I made a pair of rods to adjust the springs. I used 18" pieces of rebar, turning them to the right diameter. Hardest part is standing on the ladder 12' in the air with these ****** springs "right there" and, loosening the set screws and turning those springs and then tightening them again.
Easy, yes, but nerve racking no matter how you look at it. You MUST pay attention to every move you make -- and be capable of doing so. Also, make sure you count the number of turns by counting the spiral rings of paint. These make sure you balance the left and right spring tensions.