My baby is underweight

Hi mommies just wanna ask mcm mana nak naikkan berat badan baby? My baby now 6m+ the weight only 6.4kg, previous month 6.1kg. Tkbnyak naik. So my baby now in the middle of border line to yellow line ? dia fully bf but i want give formula milk too.Just want to know how can i mix the fm with breast milk?