My boxwood looks terrible. How do I care for it?

This looks like environmental damage. It is probably not a disease. Prune out the dead branches and give it some fertilizer and it will grow out of it.
​
Related Videos:
Boxwood
What’s wrong with my boxwood? Some leaves are yellowing.

Related Resources:
Boxwood
Boxwood Culture and Diseases - Including Boxwood Blight
Pruning Boxwoods