A lift cable under tension is not a part to learn on. The drum it wraps around is spinning against spring force, and if the cable lets go while you're holding a wrench near it, that energy has to go somewhere. We don't walk homeowners through this one over the phone, and we won't send a video link instead of a truck. If the door is still moving, even badly, leave it alone and don't run the opener. If it's already down and won't budge, that's usually the cable or the drum it rides on, and it's a one-visit fix once we're there. The honest limit here: if the cable failure has already let the door drop and bend a track or crack a panel, the cable job turns into something bigger, and we'll tell you that before we start rather than after.