To do that: 

Go to 'Menu' > 'Settings', tap 'Battery' and then switch to 'Battery Safer' or 'Low Performance'. Remember to restart the game which allows the change to take effect. 

High Performance gives you the best animation effect and smooth gaming experience. But it also drain the battery fast. Not recommended for low memory devices. 

Battery Safer gives you acceptable animation effect and fairly good gaming experience. It saves your battery in an efficient way. 

Low Performance will significantly reduce the animations played in your game, in order to improve battery life and your overall gaming experience. Use this option if your device has lower memory or CPU power.