How do you lose weight in the bust?

Sorry, but you cannot "spot reduce". Fat is lost proportionally through the entire body, all at once. You can strengthen the chest muscles and that will help support the breasts better, but will probably make them a bit bigger in the process. The only way to lose size in the bust is to just lose weight in general, but breast tissue is not fat, so even losing weight might not reduce the size of your breasts.