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Please expand the question. Please include details...what kind of tree, are the trees inside or outside, how many leaves (percentage) are brown, do the leaves have brown edges or is the whole leaf brown, has the tree had sufficient water, light and nutrition. Need more information.

Need more information.
What plant is it?
How long after purchase did you notice symptoms?
Is it in a container or planted into the ground?
How much and how often does it get water?
Have you seen pests?
Have you contacted the seller?

I think that it would have a better chance outdoors than in the basement. Place it near the foundation of your house and then cover the base with leaves or straw and then wrap the entire thing in burlap or an old blanket. Do not use plastic. I can't guarantee it will survive, but this will give it the best chance.

Just trim any new growth. Sealing the trunk can result in trapping disease and fungus against the step, which could kill the tree.

Wait, flowering allows for new growth. Heres an article about pruning boxwoods;
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm

Your local cooperative extension office may be able to help or have connections to people who can help: https://www.gardeningknowhow.com/extension-search/