Granted garage door spring installation work requires time and attention so it is best to save the task for the afternoon of a free day.
How to install garage door extension springs.
How to replace a garage door extension spring.
The easiest way to do this is to take the broken spring to a local garage door dealer or home improvement center so they can help you find an exact match.
Do not try to remove the extension spring that is not broken when the garage door is closed.
Since extension springs are determined by how much weight they each lift and the pulley system cuts this in half you need to replace your old springs with 200 pound extension springs.