What can a 72 year old woman do to improve sagging breasts due to yo yo weight loss over many years?

Without a picture, it would be difficult to be sure, but I think you would probably require a breast lift (mastopexy).  The larger issue is whether you should undergo that procedure. In other words, are you a young, healthy 72 or do you have medical problems such as heart disease, poorly controlled high blood pressure, etc.  Your plastic surgeon will be able to guide you through the process of deciding.  Good luck.