Hello. My name is***** to JustAnswer. I'm reviewing your question now, and will post back with your reply momentarily.
What search engine would you like to make yahoo your search engine? Chrome, firefox etc?
Can you explain 'not functioning' properly and in what browser?
This link will show you how to put in yahoo as default for any browser:
Now, can you log into your yahoo email in a different browser and tell me if it is 'having issues'