If you intend to breastfeed exclusively, keep it up and feed as often as possible, as long as possible. You can also buy Human Milk Fortifier (there's Similac brand sold at some big Guardians) which is a powder you can add to your own expressed milk to help baby grow faster. Or if you feed formula milk, try Similac NeoSure which is intended for low birthweight baby. Good luck! (Mine was 2kg at birth, did all of the above and she caught up very fast)
Feed on deman if you are breastfeeding