How do I fix a flat philtrum? (Photo)

A front face photo would have been really helpful. What I can say after several decades of treating problems in this area is that a less robust, cohesive filler, such as Belotero Balance, properly injected in experienced hands, can work well for restoring a flattening philtrum or even creating one where one does not naturally exist. You would be wise to consult a board certified cosmetic dermatologist. Best of luck.