How to wean off BM

Mummies, it's been a struggle lately as we have been trying to re-introduce Formula (NAN) back to our baby who just turned 1 years old. The term re-introduce as we mix fed him when he was still up to 7 to 8 months. But after that he was fully on BM. But right now we have decide to try weaning him off breastfeeding after a year of successful journey. He doesnt want to accept his formula milk even full cream except for BM. Would appreciate suggestions/tips etc to help our baby have a smooth transition. #advicepls