iPhone stuck in old Apple-ID email

I have changed my apple-ID email online. But when I connect my iPhone to iTunes and syncronize it, it´s still stuck with the old apple-ID email! So basically, I can´t download any apps from my iPhone, because it´s somehow locked to the old apple-ID email. It´s the same with ICloud, it can´t update or anything. I have tried to reset the entire iPhone, but it still won´t upgrade to the new apple-ID email. When I log in online, it´s with the new apple-ID email and the new password, and in iTunes store it´s also registrated with the new info. So why won´t my iPhone do the same? Thanks for all replies!