I was wondering if I could Implement in-app update for my kivy app that checks for new update on start-up and instead of downloading the whole new apk file, it just fetches the changes in files and replaces old files with its new version. Should I unpack the default private.mp3 file, replace the desired files and pack it again? Or such concept is totally invalid?
This should be technically possible. The private.mp3 file is already unpacked automatically when the app is first run, so you can just modify the extracted files. It won't be unpacked again unless an APK is reinstalled, in which case your code would presumably re-download the updated versions anyway (or you can cache the changes locally).
This could perhaps be considered undesirable from a user perspective, as it introduces a security hole if your update source is compromised.