ELI5 how to target a different Android version

I wish to build against API version 19 (Kitkat) rather than the default version 23. How do I achieve this ?

After you add the Android platform, change the target version in platforms/android/AndroidManifest.xml

You should see a line that looks something like:
<uses-sdk android:minSdkVersion="16" android:targetSdkVersion="22" />

Change it to something like
<uses-sdk android:minSdkVersion="16" android:targetSdkVersion="19" />

I want to add, that even if the target sdk is set to 23, it will still run on version 19 without any issues.