turn your frown upside-down

Turn your frown upside-down is another way of describing the way someone is smiling and acts as a literal term. If you think of a face when someone is frowning, the lines of their lips are literally turned down. By telling someone to turn their frown upside down, it is another way to ask them to smile because when they smile, the lines of their lips will turn upside down, which will be up in the form of a happy expression.