Can you tell me how i can overcome undereating?

It would help: If you could be more specific as the reasons why you or someone else is undereating. Some of the nuances of how to address these issues very much depend on if it's related to a psychiatric issues or other medical issues like cancer, strokes, etc. Usually, at least the addition of supplementary meal drinks/shakes like Ensure or Boost can be a reasonable place to start.