How to Set the Version of your Android App

android:versionCode — An integer value that represents the version of the application code, relative to other versions.

1. In the AndroidManifest.xml file, add the following line between the manifest tags

<manifest xmlns...
    android:versionCode="1"

</manifest>

2. Compile and run!

Resources:
http://developer.android.com/tools/publishing/versioning.html