A cable under tension from a wound torsion spring can whip hard enough to cut skin or snap a finger if it lets go while you're working near it. That's the honest limit here: we don't talk anyone through cable replacement over the phone, and we wouldn't recommend a video tutorial either, because the spring side of the job is where people get hurt, not the cable side.
What you can safely check yourself: look at the bottom bracket where the cable attaches and see if strands are splayed out like a frayed rope end, versus a clean snap near the drum. That tells you whether it's wear (replace soon, not urgent) or sudden failure (door is stuck, call now). Beyond that inspection, leave the drum, the winding cone, and the spring itself alone.