So there's two practical ways to go about this:
When a new version drops, unzip it and drag the zipped game files into your old installation, overwriting anything it asks you to. Like, I just have a folder titled Eternum, each new version I extract the zip and drag all the actual gime files into it and overwrite the previous version.
Or, you go into the current version you have installed, inside the "game" folder, there's a folder titled "saves". Copy that folder into the new version update, also into the "game" folder.