The Android app usually has two properties called versionName and versionCode in its build.gradle (module:app) file.

Both specify the version of the app. versionName is what the user sees and can be any string.

versionCode must be an integer. Android Market (Google Play) sets the version of the application based on versionCode.

Updating an existing application is verified by versionCode. The code should begin with “1” and the value increases by one whenever you roll out a new version of your application.
When you change the version of the code, Play Store will notify the user of the update.