If you're using the Itch.io App it might be possible to just update the file you already have, but in general yeah, you do have to re-download and re-install the game with each update. I'm sorry for the trouble.
Viewing post in Our Life Build 1.2 Android Launch + Small Bug Fixes! comments
If you're having trouble with saves it might be possible to fix that if you rename the new file to something like "ourlife.apk-101-universal-release", "ourlife.apk-101-universal-release_DLC1", "ourlife.apk-101-universal-release_DLC2+Voice", or whatever version your prior build was. Most people should keep their saves automatically, though.