I don't know if you have a West Marine in your area. If you do, they most always have a nico-press tool near the area where they sell the wire. They are happy to let you use it in the store and will usually help you if you need it. A better solution may be to seek out a local rigger to replace the cables for you. If there is no one in your area just pack them up and send them to a rigger and they will duplicate them and send them back.
You notice that I am using the plural. IMO you should replace both cables unless you have already replaced one. If there are signs of wear or broken strands on one and they are the same age then it would behoove you tackle the whole job at the same time.
While you are crawling around in the bilge you should probably lube any of the sheaves (rollers) the wire travels around and generally check out the whole system.
A good way of performing this task is to attach a messenger line to the old wire and pull it through while removing the worn out cable. It may make it easier to reinstall.