Replace tension cables on drums

I assume you are referring to the coil type counterbalance tension spring and cable drum assembly on a typical household garage overhead door? If so I strongly urge you to call a professional. I've watched them replace a spring on one side of my insulated double garage door recently and the preload must be set correctly based on the weight of the door in question. It was originally professionally installed and one of the springs failed after 25 years of service. There are color coded markings on the springs that indicate what weight range the springs will handle and they wrap them to a number of turns based on a calculated table that is not trivial. This is done so that the door is correctly balanced, not too much or too little assist from the spring lift and drum cable assembly. If one or both cables have come off the drums, the springs will need to be released and correctly retensioned as the were originally (assuming they were done correctly before). These spring assemblies obviously have enough stored kinetic energy to cause severe injury if handled carelessly. You need two correctly sized tommy bars to release or retension the springs but I would never attempt it for a situation like the above especially if I had never even watched the procedure before.