I see that it requires an application for only computers. I would merely delete the old ver. and install the next one but it is a visual novel. My save would be deleted.
For android, every time you build your project, make sure to use the same package name. It looks something like “com.mycompany.mygame”. Somewhere on the metadata, there is a variable that determines the version code, called “versionCode”. Every time you make a new version of your game, increment that value.
On a device, when a new application is being installed, android looks at the package name and the version code. If the package name exists, android knows that this app is already installed. If the version code of the new build is smaller than the installed one, the user gets a message like “A newer version of this app is installed” and refuses to continue the installation. If the version codes are the same, the user gets a message “The app is already installed” and will not allow the installation to continue (probably what you are experiencing).
If the version code of the new build is bigger, then the user will get the message “Update existing app?” and will be given the option to update their app. This does not erase any saved data by the app.
For more information, you can check Android Developers - Version your app.
I hope that answers your question. There is a similar functionality for iOS apps, but I’m not familiar enough to provide details.