If you are using iOS 13 on your Apple device and you are over the age of 13, you will be able to connect your current account to Apple Sign-In.
In order to do so, you will need to open your in-game Settings Menu by tapping on the Level icon on the left side of your in-game screen.
Once you open up the Settings Menu, you can find the option ''Link to Apple''. Simply tap on it and your account should then link to Apple Sign-In.

Note that this will only apply if your Apple Sign-In Credentials are not already linked.