I assume the chain on the top drives the steel rod to turn which in turn pulls the cable up and that is what makes the door go up. Correct? correct
How dangerous is the cable? Somewhat dangerous, it's under load
Should i change it right away and is it something i can do myself? depends on the condition of the cable & depends on your skill levle. What I can tell you is that you have to relax the spring(s) before you do anything and that's the dangerous part.