Should I give my 13-month-old soy milk or cow's milk?
Your question has two parts. First, is soy milk better than cow's milk? Generally it is not, unless your child is allergic to milk. Many of the things you have been hearing about the dangers of cow's milk are probably not accurate, so check specific information with your pediatrician or a qualified nutritionist. Cow's milk is the best source of calcium, phosphorus, and vitamin D as well as protein and a few other essential nutrients for the child older than 1 year. If you are concerned, for example, about hormones given to cattle (which have not been shown to be a problem, contrary to rumors), purchase milk that says hormone-free on the label. And keep in mind that children can be allergic to soy just as they may be allergic to cow's milk. If you decide to avoid cow's milk, be sure to use a fortified soy milk with added vitamin D rather than merely a soy drink.
The second part of the question is: How do you wean a child from the breast to some form of milk? The basic answer is "gradually." Your child should be able to use a sippy cup by now. Start by eliminating one daily feeding from the breast, substituting milk from a cup. After a week, eliminate another feeding. Both you and she will be more comfortable that way. Leave the bedtime and early morning feedings for last. You can express a little milk from your breasts just for your comfort, but do not pump your breasts empty or you will continue to make milk.
My favorite weaning story is the 3-year-old who kept insisting on little snacks from his mother until she finally dried up. One day he lifted her blouse, attempted to suck, looked confused, and then said, "Doesn't work!" That was the end of it.