You've probably heard that nothing lasts forever and that might be doubly true for something like your garage door springs. Over time they will wear down, because they're under constant tension. Changes in the weather can also have a detrimental effect on the springs. Properly installed garage door springs can last a long time, even decades, but eventually they will fail and break. In some cases, they can be repaired, but in others, they must be replaced.
Replacing them yourself is a lot cheaper than having them professionally replaced, because you are cutting out the labor costs, which can be many times the cost of the materials. That is not to say that replacing them yourself is easy. It can be difficult even determining the size of the springs you'll need to replace your old ones and it can be dangerous. If you have any doubts, hire a professional to replace them for you. Nevertheless, it can be done by a homeowner and this is what you'll need to do.