change so's tune meaning

• [American slang]
  to change the manner of a person, usually from bad to good, or from rude to pleasant.
    The teller was most unpleasant until she learned that I'm a bank director. Then she changed her tune.
    "I will help change your tune by fining you $150," said the judge to the rude defendant.

• change one's tune:    1. To change one's ...
• change someone's tune:    [American idiom]to ...
• in tune with so or sth:    [American slang]1. ...