If you play as a guest and you want to register in order not to risk losing your data, you must access the game once again as a guest and, once you have entered the game, click on the User Options button (indicated by the red arrows in the image below)

then you must click on Online (indicated by the red arrow in the image below);
then click on the Complete button (indicated by the blue arrow in the image below);

Then enter your Email and Password in the screen that appears (example below)

and click on the green "OK!" button