How did Jesus cure the lunatic in Matthew 17?

By rebuking the devil

And when they were come to the multitude, there came to him a certain man, kneeling down to him, and saying,

Lord, have mercy on my son: for he is lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water. ...

And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour. Matthew 17:14-18