If you wish to reset your game and start over from the beginning, go to the in-game Settings menu and select the "Delete Account" button.
This will take you through the multi-step confirmation process of deleting your game data.
An image of an arrow pointing to the Delete Account button in the bottom-left of the Settings menu.
 
Please note that deleting your account means that your level, items, all forms of progress, and in-app purchases will be erased PERMANENTLY. The deletion process is thorough and we cannot make any efforts to recover accounts that have been deleted by this method. Make sure you are confident that you want to delete your game data before doing so.