first off, make sure you have the cheat menu opened, this is done by right clicking any character portrait and clicking the "open cheat menu" button.

second, click the decisions tab

third, find the one decision called government and click it open.

fourth, there should now be a decision that allows you to change your entire realms religion to your current religion aswell as a button that changes your whole realms culture.

My empire is too big and it crashes the game every time if I want to convert culture or faith, is there a way to fix it?