That's a very important question, Warrior!

Before reinstalling the app or changing device, it is extremely important that your sync your account to either Facebook, Google Play Games (Android) or Game Center (Apple).
Otherwise, you can loose all progress, purchases and rewards earned so far.

Your account progress can be synced and thereby saved to Facebook or Google Play Games.
You can sync your account by going to the Settings tab (gear icon) on the upper right hand corner of the main screen, then select the appropriate sync method.

Easy, isn't it?

See you on the battlefield! ;)