How do you deal with an employee who thinks they are the boss?

Take some time to understand the root cause of the employee's behavior. Once you have a better understanding of their motivations, you can approach the situation with greater empathy and tailor your approach accordingly. Once you have a better understanding of the employee's behavior, it's important to provide them with specific feedback about how their behavior is affecting the team and the company's goals. Be clear about your expectations for their behavior and performance, and provide concrete examples of how they can improve.