You can link your account using your OS platform. Here is how:

1. Click the menu button on the top right of the screen.

2. Touch the “Gear shaped icon”, first button of the menu, and open the “Options” screen.

3. Click the “Account” button.

4. Link with your OS account in activated “Link Account” Screen.

Please be aware that if you do not link your account, your game data may be lost. So, make sure to link your account to keep your progress safe.