If the bunions are causing you pain and making it hard for you to walk etc then United Healthcare or other health insurance should cover the bunion surgery.
But if the bunions are not causing you pain or making it hard to walk then you may have to pay the cost of bunion surgery out of your own pocket.
Health insurance including United Healthcare will usually only cover the cost of healthcare or surgery if it's not cosmetic and needs to be done as a medical necessity because it's affecting you in some way.