Pablo,
Sorry this is not sooner, but when I changed mine out, I secured new & old cables together. Wrapped duct tape around joining ends to allow for better sliding it along. From there you need to pull from one end and work it past interferences from the other.
I would remove stuff (drawer frames etc.) for access where you can work it along.
Make sure you get a heavy duty cable for shifting. Don't try and save money by installing an undersized cable.....
CR