Go to settings and when you open the menu you will see the button: Store your progress - Connect with Email

If you have a guest account (and no Facebook account) you can create an account. By doing so, you can play on several devices.

If you already connected a Facebook account, you can add an email account as well.

If you already have progress on the other device, you need to select the account/progress you want to continue with:

As an extra measure, you need to confirm that you really want to continue with that specific account by typing "1234" in the input field: