Before upgrading your copy of Overseer, you need to stop the overseer services. Open the Services control panel, find the service named "Overseer", and stop this service.
Once the service is stopped, you can back up the existing Overseer database data. Navigate to c:\program files\overseer\, and save a copy of overseer.db, overseerdata.db, and overseersecure.db to a different folder.
Once backed up, you can now begin the upgrade process. Ensure that you have already downloaded the most recent version of Overseer from Sensible Software Solutions (http://www.overseer-network-monitor.com/).
Double click on the overseer.exe installer to begin the upgrade process.
The upgrade installation relies on the same user data that the new installation does. You'll want to define the same folders and shortcuts you used for the initial installation.