Pruning isn't necessary on these - if you want to reduce the size of the plant, you can dig it up, split the clump and replant a smaller section - in Spring, not now. This won't affect the height though, but as Sundowner is a fairly short Phormium, I can't imagine that's a problem.

Dead leaves can be removed anytime, but I would selectively pull those out, not shear the plant back!