You need to uninstall the game first!
Remember that if you don't export your save you will lose it. To do that follow those steps:
1.Open your old game.
2.Main menu => Load => To File
3.Move the file where you want to.
4.Uninstall the old version
5.Install and open new game's version.
6.Main menu => Load => From File(And select the exported save)
If after uninstalling you still can't install it. Make sure you have enough space on your device. It needs more than the apk size!