A torsion spring is not a DIY repair. It's wound under enough tension to break a bone or worse if the winding bars slip, and that risk doesn't go down because the spring is old - if anything it goes up, since a forty-year-old spring can be more brittle and less predictable under load. We'd rather tell you that plainly than have you find out the hard way.
What you can reasonably check yourself: whether the door is binding at one specific point in the track (suggests a bent track or bad roller, not the spring), whether the cable looks frayed at the drum (replace before it snaps, not after), and whether the opener is straining audibly before the door starts moving (early sign the spring is losing tension and the opener is picking up the slack it shouldn't have to).
If you're handiest around the house, lubricating hinges and rollers with a garage-door-specific lubricant, not WD-40, is safe and worth doing twice a year. Past that - springs, cables, anything that's wound or under load - it's not worth the trip to the ER to save a service call.