Down syndrome is not a disease; it can't be treated. It is a genetic mutation that happens when the cells of the fetus are dividing. The child ends up getting 3 of Chromosome 21.
There are ways to help a child with Down syndrome to function better. For instance, it is very important to work on oral motor skills when a child is born with Down syndrome; they have very low muscle tone and that is why their tongue usually sticks out; their muscles in their mouth aren't strong enough.
All in all, there is no way to "treat" Down syndrome; it is not a disease - it is a gift :]
(Also see Related question.)