Certified GKH Gardening Expert
You can trim at any time unless it is late Autumn. It will be too cold to recover.
You will also want to correct the issue before pruning. Doing a heavy prune on top of an issue will almost be guaranteed to cause its death.
If you can include photos, this will help me to see what is going on so that I can recommend the best solution.
In the meantime, this article will give you more information on when to prune these: https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm