It is not possible to turn buttermilk into regular milk. Structural changes take place in the milk when it changes from regular milk into buttermilk, and there is no way to change it back.
Milk turns into buttermilk when the milk begins to sour. When this happens, the sugar in the milk turns into lactic acid. This creates the sour flavor of buttermilk. Traditional buttermilk occurs naturally in unpasteurized milk over time. Manufacturers make modern, pasteurized buttermilk by adding lactic acid bacteria to milk. Home cooks often add lemon juice or vinegar to regular milk to create buttermilk for recipes, but there is no way to change buttermilk back to regular milk.