You switch depending on the type of formula your baby uses. If your baby is on cow milk based formula then switch to something similar or if it is soy based formula then do the same. If your baby appears gassy of spits up, you should see your pediatrician. In the first days of the change, mix the formulas as you feed just so the baby has time to adjust. You have to remember that the baby makes the final decision depending on the bowel movements and the baby's tastes. For more information, visit http://www.ehow.com/how_2054168_switch-baby-formula.html.
If your asking how to switch your baby to a different formula, what i did for my babies was I would put half of their old formula and half of their new formula together. Give it to her like that and if that seems to work keep adding the new formula until it's just the new formula you are giving her.