How do you switch mario to luigi in the game super mario galaxy?

When you unlock luigi by beating the game with mario and having 120 stars, go to your file and click the button with mario and luigi's faces to either play with mario or luigi.How do you switch mario to luigi in the game super mario galaxy?
you have to get 120 stars. and then maybe you have to beat the game again not sure. then select your file and you should be able to pick between luigi or mario. when you get 120 stars with him you can get a 121st starHow do you switch mario to luigi in the game super mario galaxy?
You have to unlock him.

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