Knock sensor went out, but where is it?

My forester is an automatic and it isn't shifting right. Sometimes will bog way down and not want to shift down, but others it will over rev and will shift at about 5-6K, I have had to let of the gas to let it shift, but I need to just get it fixed. Also I need to know how to find it and how to uninstall the old one and Install a new one, And where i can find a new one.