You'll need to pull some of the cable through the housing and cut the housing with a decent pair of dikes
. The housing is coiled steel with a vinyl sheath, as I'm sure you've found out!
If your cable has a stop on it, you'll need to cut it off so you can pull the cable through enough to shorten the housing. Of course, you'll have to make a new cable end, or ferrule, as well. Norm's method works well: