There's a rumour which I've not yet tried myself, that with hebe and certain other shrubs which have 'lazy buds' and go bare in the middle while vigorous at the tips, that you can rejuvenate-prune them but it absolutely has to be while they are in full bloom.
If you wait until the flowers are going over it will NOT work.
So no promises, but the idea is that you hard-prune it back to sticks of the height you prefer, while it's looking fabulous, and then sit back and wait for 2 years. It should be green and healthy again by this time next year, and flowering again the year after.
This info came from a tutor at an agricultural college, so should have at least a grain of truth in it, but he also said that unless it's a treasured plant for sentimental reasons, you might prefer just to buy a replacement...
So I tried it this May on some camellias, and am waiting for regrowth; no verdict yet!

Personally I'd prune it in the spring down to a more suitable height. I've done it at that time in the past and not managed to kill it.