Cables on a torsion door are under real tension even with the door closed, and on an extension-spring door of the age common in Greece, the springs themselves store enough force to do damage if a cable lets go mid-adjustment. This isn't a job for a ladder and a Saturday afternoon.
What you can safely do yourself: look at the cable where it wraps the drum for fraying or rust, check that the door is level side to side, and stop using the opener the moment it sticks or drops unevenly. Beyond that, leave the drum, the spring, and the cable itself alone. We'd rather answer a question over the phone than come out to a job that got worse because someone tried to wind a spring back by hand.