We transited to whole milk at 12 months. Slowly began replacing feeds with milk instead of formula. Luckily my son liked it right away.

Slowly start mixing it with the formula. We’re on soy milk at my house due to intolerance. But just start. 4 oz formula 2 oz milk. Do that for about a week. Then do half and half for a week or so. And then 2 oz formula 4 oz milk. Until it’s all milk and no formula

Start by adding normal cows milk in the breakfast of a morning instead of formula. Once that’s done for a few days switch one bottle of formula for cows milk and so on