Press around the nipple.
Gently pressing around the nipple with one hand and gently pulling the nipple with the other hand can improve mild nipple invagination.
Use nipple aspirator
If pressing and pulling doesn't work, you can try to attract the nipple with a "nipple aspirator" and wear a nipple protection cover to prevent it from being pressed in again.
Surgical correction
If the above methods cannot be improved, surgical correction can be considered. But the specific situation needs more detailed description or local photos to judge.