Not very much - if you cut back into brown branches with no foliage, they won't produce new foliage there. If they are too large for what you want, remove them and replace with something else.
If cut off significantly lower, where trunks several inches in diameter a little regrowth will occur here and there, might even eventually produce a fairly good new covering over a period of many years. In the meantime, you will be looking at stumps with greenery below, for the most part. Like bald-headed friars standing in a row, with hair only on the sides.
