hi bob i changed the cables on my henderson garage door 3 weeks after my pr£ck builder on my extension fitted them out of vertical. (dont say get him back, because when you lose confidence in someone, especially a bull ****ter, then enoughs enough)
henderson wanted £32 for a pair of cables, and refused to change them on warranty, because the only way they break is by being poorly fitted. it did,nt snap, it just went over the edge of the spool the cable is supposed to wind onto and snapped the edge of the spool.
the job you need to do, as already stated, is to take off the tension with a lever/small bar and insert a crook pin in a hole to hold it slack while you take off the wire and if necessary the spool.also check your door for vertical in case its out and has contributed to the problem
as its not a job you do often i would,nt ask a mate to stand holding the tension, as it will take a few minutes but once you have done it you will be surprised how straightforward it is
a crook pin is like a circlip or the top of a shepherds staff. if you look on ebay for cable sets you will see them in a pack of cables. you might make your own with an adapted nail
would,nt be surprised to see a demo of the job on youtube, pretty good that way