You can tap on the Home button at the upper left hand corner from both the boosts menu and the tally screen at the end of a turn.
You can also use the pause button to exit the game. Tap on the Pause button at the top left hand corner of the game screen and choose the red X to exit to the Home screen.
You can also use the pause button to exit the game. Tap on the Pause button at the top left hand corner of the game screen and choose the red X to exit to the Home screen.