Praise be to Allah.
If the sagging is extreme and looks ugly, and causes psychological distress, and this surgery is safe and is thought most likely to be beneficial, then there is nothing wrong with having this surgery to lift the breasts and resolve the sagging. This comes under the heading of removing defects.
But if the sagging is normal, as usually happens to the one who breastfeeds, it is not permissible to have surgery to resolve it, and that comes under the heading of prohibited beautification.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) was asked:
What is the ruling on cosmetic surgery that women undergo?
He replied: If it is to remove a defect, there is nothing wrong with it, so long as it will not lead to any harm, because ‘Arfajah (may Allah be pleased with him) wore a nose made of gold when his nose was cut off [in battle].
An example of that is lifting or tightening sagging breasts."(Thamarat at-Tadwin p. 133).
See also the answers to questions no. 108860 and 47694 .
And Allah knows best.