Procedure Fix Tuberous Breasts on a 20 Year Old?

Your breasts do not appear to be truly tuberous judging from the photo. They are sagging and wide set, but probably not tuberous. 

It looks like many of us agree that you have a significant amount of ptosis (nipple/areola complex below the breast fold and pointing downward).  You would need a breast lift procedure for better placement of the nipple/areola on the breast mound and if you want to increase the volume of your breasts, breast augmentation would need to be performed.

Anyone considering plastic or reconstructive surgery should consider consulting a board certified plastic surgeon to achieve the best and safest results possible. Physicians who are considered board certified hold a certification with the American Board of Plastic Surgery (ABPS). This certification proves that the surgeon has graduated from an accredited medical school, has completed a residency of at least five years, has three years of experience in general surgery, is actively practicing plastic surgery for at least two years, and has passed mandatory comprehensive and written examinations. These surgeons are referred to as diplomats of the ABPS and have fulfilled the following requirements. 
It is in the patient’s best interest to choose a board certified plastic surgeon. It is vital to your safety and the success of your cosmetic surgery that the surgeon you choose has the right qualifications, ample experience, and documented results of his or her work. ABPS diplomats have extensive skill and experience in a wide range of plastic surgery procedures.

Best Wishes

From your pictures. it appears that you may have a grade 3 ptosis and would likely benefit from a full mastopexy with breast implants if you desire upper breast fullness.