Do I have to wait until I get enough points to change again?
No, like ST said, you can change anytime your citizens are not content. You will
want to wait until you have enough points to select a new SP (and select that new SP) because in addition to losing two ideology tenets, your accumulated culture is reset to zero.
I have never played BNW at Prince, but I guess I am very surprised that you are facing pressure to switch ideologies at Prince.
So what happens to all those points you accumulated to be able to change in the first place?
It is not points so much as picks. Suppose, for example, you have three tenets in Freedom. After switching, you will have one pick in either Order or Autocracy.
Or maybe my question should be, how can I get the social policy screen to show up?
Click on the usual SP icon at the top of the menu that shows you all the SP policies. From that screen, click the link/button in the ideology table. From that screen at the bottom there may be a red frowning face showing that your citizens are not content. If so, there is a text link with the option to switch ideologies -- or pop-up text saying that you cannot switch because your citizens are content.
Don't switch unless the red number is like 20 or more as you may find yourself in the new ideology, with two fewer tenets, but even more unhappy citizens! You do not need EUI.