How do I reform my religion in Crusader kings 2? Right now I'm Pagan. I'm playing as Greater Poland in the DLC Old Gods.
Conquer 3 of your religion's 5 holy cities, get Religious Authority in your own religion to at least 50, make sure you have 750 piety and then hit the reform button on the religion screen.
Alternatively, if the moral authority or piety requirements are too hard you can instead control all five holy sites - this is also sufficient to reform.