Once you have login to Google Play Developer console, click:
All aplication --> Killah Zombie --> Release Management --> App Releases ---> Create Release
Then upload yout new .apk, write in your update notes, update number, ect.
Also, while exporting .apk from AppGameKit, your 'version number' must be higher than the previous uploaded .apk version number.
You also need to have the same keystore file and password as the first .apk that you uploaded to Gogle Play
Then, save, review your rollout, then finish it. Then wait around 1 day for the new update to be reviewed and released by Google.
APK update is separate from your Google Play 'Store Presence' update.
Google Play store lsitings need to be updated if you want to upload new screenshot, new description, ect.
APK update is just the software. Store Presence will affect how people see your app page in Google Play store.