FTM introducing milk help please 🍼

My 10 month old doesn’t want her formula much anymore. She never finishes a bottle like she used to. Her doctor said to start to give her 3.25% milk and as much formula as she will drink (up to 20 ounces a day). Do I give her the milk by itself or can I use it to mix the formula instead of water? Or can I do both? Help please! Thanks in advance 😁