What happens when a fight breaks out between passengers,when the plane is already in the air?

According to Business Insider, the first move is to try to separate the parties involved.

Flight attendants try to calm and reseat the warring parties, and ask for assistance from other passengers if needed. The main key is to deescalate the situation.

That may be accomplished by imply speaking with the passengers and letting them know what may happen. For example, landing at the nearest airport or being arrested once on the ground!