There isn't any tips to decrease weight of baby. You are about 2 months away to due. Maybe just lessen of pregnancy milk or sugary stuff. Alternatively to eat in moderation. The baby's weight given are just a plus minus. Hence don't worry much. But, I guess your gynae advice you so in order to allow you to have smooth natural vaginal birth with minimal tearing (if any and atleast) because if baby's too huge be it based on weight or structure, you may not be able to do vaginal birth.