You have two possibilities to cancel a started game and start a new one.
1. With the Play button in the menu. Open the menu (flat arrow), tap the Play button, and confirm your choice with OK.
2. Via the statistics screen. Open the menu (flat arrow) and select the statistics (chart icon). Now tap on "Delete" under the statistics chart and confirm with OK.
You can start a new game by tapping on the menu (the three horizontal lines in the upper left corner) and then selecting the second option "New game".