This guide will detail the steps required to upload and overwrite the application provided to you in .apk file format for a new version.
1. Login to Google Play to access the developer tools:
2. On the list of your applications, click the application you would like to update
3. When viewing the application overview screen, click the three small bars in the top left corner and select the APK option
4. Click Upload New APK To Production
5. Click Browse, selecting the APK of your updated application
6. Upon completion of the upload, click Publish Now To Production
7. Finally, select Publish in the top right hand corner and then select Publish This Version to allow the Google Play listing to update with the latest version to download for all users.