[iOS]
Please contact Apple for enquires relating to re-downloads or support.

Apple Support
http://www.apple.com/support/

[Android]
You will be able to re-download previously downloaded apps free of charge from GooglePlay's My Apps.

*Please be sure to perform a data backup through the 'SQUARE ENIX BRIDGE' option before deleting the app. Otherwise you will lose your game data. You can find this option in 'Backup Data' in the 'Other' menu.