I am 18 years old. My bra size is 38D. And my breasts are very saggy. I am overweight (BMI of 26.5) (not obese). How can I make them firm?

I am 18 years old. My bra size is 38D. And my breasts are very saggy. I am overweight (BMI of 26.5) (not obese). How can I make them firm?
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